Peter Dahlberg

All Publications

• Time-resolved cryogenic electron tomography for the study of transient cellular processes. Molecular biology of the cell Yoniles, J., Summers, J. A., Zielinski, K. A., Antolini, C., Panjalingam, M., Lisova, S., Moss, F. R., Perna, M. A., Kupitz, C., Hunter, M. S., Pollack, L., Wakatsuki, S., Dahlberg, P. D. 2024: mbcE24010042

  Abstract

  Cryogenic electron tomography (cryo-ET) is the highest resolution imaging technique applicable to the life sciences, enabling sub-nanometer visualization of specimens preserved in their near native states. The rapid plunge freezing process used to prepare samples lends itself to time-resolved studies, which researchers have pursued for in vitro samples for decades. Here, we focus on developing a freezing apparatus for time-resolved studies in situ. The device mixes cellular samples with solution-phase stimulants before spraying them directly onto an electron microscopy grid that is transiting into cryogenic liquid ethane. By varying the flow rates of cell and stimulant solutions within the device, we can control the reaction time from tens of milliseconds to over a second prior to freezing. In a proof-of-principle demonstration, the freezing method is applied to a model bacterium, Caulobacter crescentus, mixed with an acidic buffer. Through cryo-ET we resolved structural changes throughout the cell, including surface-layer protein dissolution, outer membrane deformation, and cytosolic rearrangement, all within 1.5 seconds of reaction time. This new approach, Time-Resolved cryo-ET (TR-cryo-ET), enhances the capabilities of cryo-ET by incorporating a sub-second temporal axis and enables the visualization of induced structural changes at the molecular, organelle, or cellular level. [Media: see text].

  View details for DOI 10.1091/mbc.E24-01-0042

  View details for PubMedID 38717434

• Exploring transient states of PAmKate to enable improved cryogenic single-molecule imaging. bioRxiv : the preprint server for biology Perez, D., Dowlatshahi, D. P., Azaldegui, C. A., Dahlberg, P. D., Moerner, W. E. 2024

  Abstract

  Super-resolved cryogenic correlative light and electron microscopy is a powerful approach which combines the single-molecule specificity and sensitivity of fluorescence imaging with the nano-scale resolution of cryogenic electron tomography. Key to this method is active control over the emissive state of fluorescent labels to ensure sufficient sparsity to localize individual emitters. Recent work has identified fluorescent proteins (FPs) which photoactivate or photoswitch efficiently at cryogenic temperatures, but long on-times due to reduced quantum yield of photobleaching remains a challenge for imaging structures with a high density of localizations. In this work, we explore the photophysical properties of the red photoactivatable FP PAmKate and identify a 2-color process leading to enhanced turn-off of active emitters, improving localization rate. Specifically, after excitation of ground state molecules, we find a transient state forms with a lifetime of ~2 ms which can be bleached by exposure to a second wavelength. We measure the response of the transient state to different wavelengths, demonstrate how this mechanism can be used to improve imaging, and provide a blueprint for study of other FPs at cryogenic temperatures.

  View details for DOI 10.1101/2024.04.24.590965

  View details for PubMedID 38712218

  View details for PubMedCentralID PMC11071506

• Advanced Cryogenic Light Microscopy Stage to Enable 3D Super-resolved Cryogenic Correlative Light and Electron Microscopy. Microscopy and microanalysis : the official journal of Microscopy Society of America, Microbeam Analysis Society, Microscopical Society of Canada Perez, D., Dahlberg, P. D., Moerner, W. E. 2023; 29 (Supplement_1): 1941

  View details for DOI 10.1093/micmic/ozad067.1005

  View details for PubMedID 37612900

• Characterization of mApple as a Red Fluorescent Protein for Cryogenic Single-Molecule Imaging with Turn-Off and Turn-On Active Control Mechanisms. The journal of physical chemistry. B Sartor, A. M., Dahlberg, P. D., Perez, D., Moerner, W. E. 2023

  Abstract

  Single-molecule superresolution microscopy is a powerful tool for the study of biological structures on size scales smaller than the optical diffraction limit. Imaging samples at cryogenic temperatures (77 K) reduces the quantum yield of photobleaching for many fluorescent labels, yielding localization precisions below 10 nm. Cryogenic imaging further enables correlation with cryogenic electron tomography. A key limitation in applying methods such as PALM and STORM to samples maintained at 77 K is the limited number of fluorophores known to undergo efficient turn-on and turn-off mechanisms necessary to control the sparsity of active emitters. We find that mApple, a red-emitting fluorescent protein, undergoes a novel turn-off mechanism in response to simultaneous illumination with two colors of light. This turn-off mechanism enables localization of many individual molecules in initially bright samples, but the final density of localizable emitters is limited by relatively inefficient turn-on (photoactivation). Bulk excitation and emission spectroscopy shows that mApple has access to two distinct emissive states as well as dark states accessible optically or through changes in pH. The bright and stable emission of mApple enables widefield collection of single-molecule emission spectra, which highlight the complex nature and environmental sensitivity of states observed in red fluorescent proteins.

  View details for DOI 10.1021/acs.jpcb.2c08995

  View details for PubMedID 36943356

• Ratiometric sensing of redox environments inside individual carboxysomes trapped in solution Carpenter, W. B., Lavania, A. A., Turnsek, J. B., Perez, D., Oltrogge, L. M., Dahlberg, P. D., Savage, D. F., Moerner, W. E. CELL PRESS. 2023: 304A

  View details for Web of Science ID 000989629701600

• Metallic Support Films Reduce Optical Heating in Cryogenic Correlative Light and Electron Tomography. Journal of structural biology Dahlberg, P. D., Perez, D., Hecksel, C. W., Chiu, W., Moerner, W. E. 2022: 107901

  Abstract

  Super-resolved cryogenic correlative light and electron tomography is an emerging method that provides both the single-molecule sensitivity and specificity of fluorescence imaging, and the molecular scale resolution and detailed cellular context of tomography, all in vitrified cells preserved in their native hydrated state. Technical hurdles that limit these correlative experiments need to be overcome for the full potential of this approach to be realized. Chief among these is sample heating due to optical excitation which leads to devitrification, a phase transition from amorphous to crystalline ice. Here we show that much of this heating is due to the material properties of the support film of the electron microscopy grid, specifically the absorptivity and thermal conductivity. We demonstrate through experiment and simulation that the properties of the standard holey carbon electron microscopy grid lead to substantial heating under optical excitation. In order to avoid devitrification, optical excitation intensities must be kept orders of magnitude lower than the intensities commonly employed in room temperature super-resolution experiments. We further show that the use of metallic films, either holey gold grids, or custom made holey silver grids, alleviate much of this heating. For example, the holey silver grids permit 20* the optical intensities used on the standard holey carbon grids. Super-resolution correlative experiments conducted on holey silver grids under these increased optical excitation intensities have a corresponding increase in the rate of single-molecule fluorescence localizations. This results in an increased density of localizations and improved correlative imaging without deleterious effects from sample heating.

  View details for DOI 10.1016/j.jsb.2022.107901

  View details for PubMedID 36191745

• Cryo-electron tomography with mixed-scale dense neural networks reveals key steps in deployment of Toxoplasma invasion machinery. PNAS nexus Segev-Zarko, L. A., Dahlberg, P. D., Sun, S. Y., Pelt, D. M., Kim, C. Y., Egan, E. S., Sethian, J. A., Chiu, W., Boothroyd, J. C. 2022; 1 (4): pgac183

  Abstract

  Host cell invasion by intracellular, eukaryotic parasites within the phylum Apicomplexa is a remarkable and active process involving the coordinated action of apical organelles and other structures. To date, capturing how these structures interact during invasion has been difficult to observe in detail. Here, we used cryogenic electron tomography to image the apical complex of Toxoplasma gondii tachyzoites under conditions that mimic resting parasites and those primed to invade through stimulation with calcium ionophore. Through the application of mixed-scale dense networks for image processing, we developed a highly efficient pipeline for annotation of tomograms, enabling us to identify and extract densities of relevant subcellular organelles and accurately analyze features in 3-D. The results reveal a dramatic change in the shape of the anteriorly located apical vesicle upon its apparent fusion with a rhoptry that occurs only in the stimulated parasites. We also present information indicating that this vesicle originates from the vesicles that parallel the intraconoidal microtubules and that the latter two structures are linked by a novel tether. We show that a rosette structure previously proposed to be involved in rhoptry secretion is associated with apical vesicles beyond just the most anterior one. This result, suggesting multiple vesicles are primed to enable rhoptry secretion, may shed light on the mechanisms Toxoplasma employs to enable repeated invasion attempts. Using the same approach, we examine Plasmodium falciparum merozoites and show that they too possess an apical vesicle just beneath a rosette, demonstrating evolutionary conservation of this overall subcellular organization.

  View details for DOI 10.1093/pnasnexus/pgac183

  View details for PubMedID 36329726

  View details for PubMedCentralID PMC9615128

• Identification and Demonstration of roGFP2 as an Environmental Sensor for Cryogenic Correlative Light and Electron Microscopy. Journal of structural biology Perez, D., Dahlberg, P. D., Wang, J., Sartor, A. M., Borden, J. S., Shapiro, L., Moerner, W. E. 2022: 107881

  Abstract

  Cryogenic correlative light and electron microscopy (cryo-CLEM) seeks to leverage orthogonal information present in two powerful imaging modalities. While recent advances in cryogenic electron microscopy (cryo-EM) allow for the visualization and identification of structures within cells at the nanometer scale, information regarding the cellular environment, such as pH, membrane potential, ionic strength etc. that influence the observed structures remains absent. Fluorescence microscopy can potentially be used to reveal this information when specific labels, known as fluorescent biosensors, are used, but there has been minimal use of such biosensors in cryo-CLEM to date. Here we demonstrate the applicability of one such biosensor, the fluorescent protein roGFP2, for cryo-CLEM experiments. At room temperature, the ratio of roGFP2 emission brightness when excited at 425 nm or 488 nm is known to report on the local redox potential. When samples containing roGFP2 are rapidly cooled to 77K in a manner compatible with cryo-EM, the ratio of excitation peaks remains a faithful indicator of the redox potential at the time of freezing. Using purified protein in different oxidizing/reducing environments, we generate a calibration curve which can be used to analyze in situ measurements. As a proof-of-principle demonstration, we investigate the oxidation/reduction state within vitrified Caulobacter crescentus cells. The polar organizing protein Z (PopZ) localizes to the polar regions of C. crescentus where it is known to form a distinct microdomain. By expressing an inducible roGFP2-PopZ fusion we can visualize individual microdomains in the context of their redox environment.

  View details for DOI 10.1016/j.jsb.2022.107881

  View details for PubMedID 35811036

• Ratiometric Sensing of Redox Environments Inside Individual Carboxysomes Trapped in Solution. The journal of physical chemistry letters Carpenter, W. B., Lavania, A. A., Borden, J. S., Oltrogge, L. M., Perez, D., Dahlberg, P. D., Savage, D. F., Moerner, W. E. 2022: 4455-4462

  Abstract

  Diffusion of biological nanoparticles in solution impedes our ability to continuously monitor individual particles and measure their physical and chemical properties. To overcome this, we previously developed the interferometric scattering anti-Brownian electrokinetic (ISABEL) trap, which uses scattering to localize a particle and applies electrokinetic forces that counteract Brownian motion, thus enabling extended observation. Here we present an improved ISABEL trap that incorporates a near-infrared scatter illumination beam and rapidly interleaves 405 and 488 nm fluorescence excitation reporter beams. With the ISABEL trap, we monitored the internal redox environment of individual carboxysomes labeled with the ratiometric redox reporter roGFP2. Carboxysomes widely vary in scattering contrast (reporting on size) and redox-dependent ratiometric fluorescence. Furthermore, we used redox sensing to explore the chemical kinetics within intact carboxysomes, where bulk measurements may contain unwanted contributions from aggregates or interfering fluorescent proteins. Overall, we demonstrate the ISABEL trap's ability to sensitively monitor nanoscale biological objects, enabling new experiments on these systems.

  View details for DOI 10.1021/acs.jpclett.2c00782

  View details for PubMedID 35549289

• A bottom-up perspective on photodynamics and photoprotection in light-harvesting complexes using anti-Brownian trapping. The Journal of chemical physics Squires, A. H., Wang, Q., Dahlberg, P. D., Moerner, W. E. 2022; 156 (7): 070901

  Abstract

  Single-molecule fluorescence spectroscopy allows direct, real-time observation of dynamic photophysical changes in light harvesting complexes. The Anti-Brownian ELectrokinetic (ABEL) trap is one such single-molecule method with useful advantages. This approach is particularly well-suited to make detailed spectroscopic measurements of pigment-protein complexes in a solution phase because it enables extended-duration single-molecule observation by counteracting Brownian motion. This Perspective summarizes recent contributions by the authors and others that have utilized the unique capabilities of the ABEL trap to advance our understanding of phycobiliproteins and the phycobilisome complex, the primary light-harvesting apparatus of cyanobacteria. Monitoring the rich spectroscopic data from these measurements, which include brightness, fluorescence lifetime, polarization, and emission spectra, among other measurable parameters, has provided direct characterization of pigments and energy transfer pathways in the phycobilisome, spanning scales from single pigments and monomeric phycobiliproteins to higher order oligomers and protein-protein interactions of the phycobilisome complex. Importantly, new photophysical states and photodynamics were observed to modulate the flow of energy through the phycobilisome and suggest a previously unknown complexity in phycobilisome light harvesting and energy transport with a possible link to photoadaptive or photoprotective functions in cyanobacteria. Beyond deepening our collective understanding of natural light-harvesting systems, these and future discoveries may serve as inspiration for engineering improved artificial light-harvesting technologies.

  View details for DOI 10.1063/5.0079042

  View details for PubMedID 35183095

• ATP-responsive biomolecular condensates tune bacterial kinase signaling. Science advances Saurabh, S., Chong, T. N., Bayas, C., Dahlberg, P. D., Cartwright, H. N., Moerner, W. E., Shapiro, L. 2022; 8 (7): eabm6570

  Abstract

  Biomolecular condensates formed via liquid-liquid phase separation enable spatial and temporal organization of enzyme activity. Phase separation in many eukaryotic condensates has been shown to be responsive to intracellular adenosine triphosphate (ATP) levels, although the consequences of these mechanisms for enzymes sequestered within the condensates are unknown. Here, we show that ATP depletion promotes phase separation in bacterial condensates composed of intrinsically disordered proteins. Enhanced phase separation promotes the sequestration and activity of a client kinase enabling robust signaling and maintenance of viability under the stress posed by nutrient scarcity. We propose that a diverse repertoire of condensates can serve as control knobs to tune enzyme sequestration and reactivity in response to the metabolic state of bacterial cells.

  View details for DOI 10.1126/sciadv.abm6570

  View details for PubMedID 35171683

• Cryogenic Super-Resolution Fluorescence and Electron Microscopy Correlated at the Nanoscale. Annual review of physical chemistry Dahlberg, P. D., Moerner, W. E. 2021

  Abstract

  We review the emerging method of super-resolved cryogenic correlative light and electron microscopy (srCryoCLEM). Super-resolution (SR) fluorescence microscopy and cryogenic electron tomography (CET) are both powerful techniques for observing subcellular organization, but each approach has unique limitations. The combination of the two brings the single-molecule sensitivity and specificity of SR to the detailed cellular context and molecular scale resolution of CET. The resulting correlative data is more informative than the sum of its parts. The correlative images can be used to pinpoint the positions of fluorescently labeled proteins in the high-resolution context of CET with nanometer-scale precision and/or to identify proteins in electron-dense structures. The execution of srCryoCLEM is challenging and the approach is best described as a method that is still in its infancy with numerous technical challenges. In this review, we describe state-of-the-art srCryoCLEM experiments, discuss the most pressing challenges, and give a brief outlook on future applications. Expected final online publication date for the Annual Review of Physical Chemistry, Volume 72 is April 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

  View details for DOI 10.1146/annurev-physchem-090319-051546

  View details for PubMedID 33441030

• Cryogenic single-molecule fluorescence annotations for electron tomography reveal in situ organization of key proteins in Caulobacter. Proceedings of the National Academy of Sciences of the United States of America Dahlberg, P. D., Saurabh, S., Sartor, A. M., Wang, J., Mitchell, P. G., Chiu, W., Shapiro, L., Moerner, W. E. 2020

  Abstract

  Superresolution fluorescence microscopy and cryogenic electron tomography (CET) are powerful imaging methods for exploring the subcellular organization of biomolecules. Superresolution fluorescence microscopy based on covalent labeling highlights specific proteins and has sufficient sensitivity to observe single fluorescent molecules, but the reconstructions lack detailed cellular context. CET has molecular-scale resolution but lacks specific and nonperturbative intracellular labeling techniques. Here, we describe an imaging scheme that correlates cryogenic single-molecule fluorescence localizations with CET reconstructions. Our approach achieves single-molecule localizations with an average lateral precision of 9 nm, and a relative registration error between the set of localizations and CET reconstruction of 30 nm. We illustrate the workflow by annotating the positions of three proteins in the bacterium Caulobacter crescentus: McpA, PopZ, and SpmX. McpA, which forms a part of the chemoreceptor array, acts as a validation structure by being visible under both imaging modalities. In contrast, PopZ and SpmX cannot be directly identified in CET. While not directly discernable, PopZ fills a region at the cell poles that is devoid of electron-dense ribosomes. We annotate the position of PopZ with single-molecule localizations and confirm its position within the ribosome excluded region. We further use the locations of PopZ to provide context for localizations of SpmX, a low-copy integral membrane protein sequestered by PopZ as part of a signaling pathway that leads to an asymmetric cell division. Our correlative approach reveals that SpmX localizes along one side of the cell pole and its extent closely matches that of the PopZ region.

  View details for DOI 10.1073/pnas.2001849117

  View details for PubMedID 32513734

• Cryogenic Superresolution Fluorescence Correlated with Cryogenic Electron Tomography: Combining Specific Labeling and High Resolution Dahlberg, P. D., Saurabh, S., Wang, J., Sartor, A. M., Chiu, W., Shapiro, L., Moerner, W. E. CELL PRESS. 2020: 20A–21A

  View details for Web of Science ID 000513023200098

• Robust Modulation of a Bacterial Kinase by Protein Phase Separation Saurabh, S., Chong, T., Bayas, C., Dahlberg, P. D., Moerner, W. E., Shapiro, L. CELL PRESS. 2020: 203A

  View details for Web of Science ID 000513023201267

• Cryogenic Correlative Single-Particle Photoluminescence Spectroscopy and Electron Tomography for Investigation of Nanomaterials. Angewandte Chemie (International ed. in English) Dahlberg, P. D., Perez, D. n., Su, Z. n., Chiu, W. n., Moerner, W. E. 2020

  Abstract

  Cryogenic single-particle photoluminescence (PL) spectroscopy has been used with great success to directly observe the heterogeneous photophysical states present in a population of luminescent particles. Cryogenic electron tomography provides complimentary nanometer scale structural information to PL spectroscopy, but the two techniques have not been correlated due to technical challenges. Here, we present a method for correlating single-particle information from these two powerful microscopy modalities. We simultaneously observe PL brightness, emission spectrum, and in-plane excitation dipole orientation of CdSSe/ZnS quantum dots suspended in vitreous ice. Stable and fluctuating emitters were observed, as well as a surprising splitting of the PL spectrum into two bands with an average energy separation of 80 meV. In some cases the onset of the splitting corresponded to changes in the in-plane excitation dipole orientation. These dynamics were assigned to structures of individual quantum dots and the excitation dipoles were visualized in the context of structural features.

  View details for DOI 10.1002/anie.202002856

  View details for PubMedID 32330371

• Interferometric Scattering Enables Fluorescence-Free Electrokinetic Trapping of Single Nanoparticles in Free Solution. Nano letters Squires, A. H., Lavania, A. A., Dahlberg, P. D., Moerner, W. E. 2019

  Abstract

  Anti-Brownian traps confine single particles in free solution by closed-loop feedback forces that directly counteract Brownian motion. Extended-duration measurements on trapped objects allow detailed characterization of photophysical and transport properties as well as observation of infrequent or rare dynamics. However, this approach has been generally limited to particles that can be tracked by fluorescence emission. Here we present the Interferometric Scattering Anti-Brownian ELectrokinetic (ISABEL) trap, which uses interferometric scattering rather than fluorescence to monitor particle position. By decoupling the ability to track (and therefore trap) a particle from collection of its spectroscopic data, the ISABEL trap enables confinement and extended study of single particles that do not fluoresce, only weakly fluoresce, or exhibit intermittent fluorescence or photobleaching. This new technique significantly expands the range of nanoscale objects that may be investigated at the single-particle level in free solution.

  View details for DOI 10.1021/acs.nanolett.9b01514

  View details for PubMedID 31117762

• Developments in cryogenic single-molecule super-resolution imaging and dynamics of photosynthetic antennas in solution Dahlberg, P. AMER CHEMICAL SOC. 2019

  View details for Web of Science ID 000478861204099

• Single-molecule trapping and spectroscopy reveals photophysical heterogeneity of phycobilisomes quenched by Orange Carotenoid Protein. Nature communications Squires, A. H., Dahlberg, P. D., Liu, H., Magdaong, N. C., Blankenship, R. E., Moerner, W. E. 2019; 10 (1): 1172

  Abstract

  The Orange Carotenoid Protein (OCP) is a cytosolic photosensor that is responsible for non-photochemical quenching (NPQ) of the light-harvesting process in most cyanobacteria. Upon photoactivation by blue-green light, OCP binds to the phycobilisome antenna complex, providing an excitonic trap to thermally dissipate excess energy. At present, both the binding site and NPQ mechanism of OCP are unknown. Using an Anti-Brownian ELectrokinetic (ABEL) trap, we isolate single phycobilisomes in free solution, both in the presence and absence of activated OCP, to directly determine the photophysics and heterogeneity of OCP-quenched phycobilisomes. Surprisingly, we observe two distinct OCP-quenched states, with lifetimes 0.09ns (6% of unquenched brightness) and 0.21ns (11% brightness). Photon-by-photon Monte Carlo simulations of exciton transfer through the phycobilisome suggest that the observed quenched states are kinetically consistent with either two or one bound OCPs, respectively, underscoring an additional mechanism for excitation control in this key photosynthetic unit.

  View details for PubMedID 30862823

• Single-molecule trapping and spectroscopy reveals photophysical heterogeneity of phycobilisomes quenched by Orange Carotenoid Protein NATURE COMMUNICATIONS Squires, A. H., Dahlberg, P. D., Liu, H., Magdaong, N. M., Blankenship, R. E., Moerner, W. E. 2019; 10

  View details for DOI 10.1038/s41467-019-09084-2

  View details for Web of Science ID 000460930600004

• Orientational Dynamics of Transition Dipoles and Exciton Relaxation in LH2 from Ultrafast Two-Dimensional Anisotropy JOURNAL OF PHYSICAL CHEMISTRY LETTERS Massey, S. C., Ting, P., Yeh, S., Dahlberg, P. D., Sohail, S. H., Allodi, M. A., Martin, E. C., Kais, S., Hunter, C., Engel, G. S. 2019; 10 (2): 270–77

  Abstract

  Light-harvesting complexes in photosynthetic organisms display fast and efficient energy transfer dynamics, which depend critically on the electronic structure of the coupled chromophores within the complexes and their interactions with their environment. We present ultrafast anisotropy dynamics, resolved in both time and frequency, of the transmembrane light-harvesting complex LH2 from Rhodobacter sphaeroides in its native membrane environment using polarization-controlled two-dimensional electronic spectroscopy. Time-dependent anisotropy obtained from both experiment and modified Redfield simulation reveals an orientational preference for excited state absorption and an ultrafast equilibration within the B850 band in LH2. This ultrafast equilibration is favorable for subsequent energy transfer toward the reaction center. Our results also show a dynamic difference in excited state absorption anisotropy between the directly excited B850 population and the population that is initially excited at 800 nm, suggesting absorption from B850 states to higher-lying excited states following energy transfer from B850*. These results give insight into the ultrafast dynamics of bacterial light harvesting and the excited state energy landscape of LH2 in the native membrane environment.

  View details for DOI 10.1021/acs.jpclett.8b03223

  View details for Web of Science ID 000456493200022

  View details for PubMedID 30599133

• NANOSCALE ELUCIDATION OF THE INVASION APPARATUS OF APICOMPLEXAN PARASITES Segev-Zarko, L., Sun, S. Y., Dahlberg, P. D., Pelt, D., Chen, J., Schmid, M. F., Galaz-Montoya, J., Moerner, W. E., Larabell, C., Sethian, J., Chiu, W., Boothroyd, J. AMER SOC TROP MED & HYGIENE. 2019: 620

  View details for Web of Science ID 000507364505127

• Identification of PAmKate as a Red Photoactivatable Fluorescent Protein for Cryogenic Super-Resolution Imaging. Journal of the American Chemical Society Dahlberg, P. D., Sartor, A. M., Wang, J., Saurabh, S., Shapiro, L., Moerner, W. E. 2018; 140 (39): 12310–13

  Abstract

  Single-molecule super-resolution fluorescence microscopy conducted in vitrified samples at cryogenic temperatures offers enhanced localization precision due to reduced photobleaching rates, a chemical-free and rapid fixation method, and the potential of correlation with cryogenic electron microscopy. Achieving cryogenic super-resolution microscopy requires the ability to control the sparsity of emissive labels at cryogenic temperatures. Obtaining this control presents a key challenge for the development of this technique. In this work, we identify a red photoactivatable protein, PAmKate, which remains activatable at cryogenic temperatures. We characterize its activation as a function of temperature and find that activation is efficient at cryogenic and room temperatures. We perform cryogenic super-resolution experiments in situ, labeling PopZ, a protein known to assemble into a microdomain at the poles of the model bacterium Caulobacter crescentus. We find improved localization precision at cryogenic temperatures compared to room temperature by a factor of 4, attributable to reduced photobleaching.

  View details for PubMedID 30222332

• Identification of PAmKate as a Red Photoactivatable Fluorescent Protein for Cryogenic Super-Resolution Imaging JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Dahlberg, P. D., Sartor, A. M., Wang, J., Saurabh, S., Shapiro, L., Moerner, W. E. 2018; 140 (39): 12310-12313

  View details for DOI 10.1021/jacs.8b05960

  View details for Web of Science ID 000446920100001

• Excitations Partition into Two Distinct Populations in Bulk Perovskites ADVANCED OPTICAL MATERIALS Wang, L., Brawand, N. P., Voeroes, M., Dahlberg, P. D., Otto, J. P., Williams, N. E., Tiede, D. M., Galli, G., Engel, G. S. 2018; 6 (5)

  View details for DOI 10.1002/adom.201700975

  View details for Web of Science ID 000426604700003

• Mapping the ultrafast flow of harvested solar energy in living photosynthetic cells NATURE COMMUNICATIONS Dahlberg, P. D., Ting, P., Massey, S. C., Allodi, M. A., Martin, E. C., Hunter, C., Engel, G. S. 2017; 8: 988

  Abstract

  Photosynthesis transfers energy efficiently through a series of antenna complexes to the reaction center where charge separation occurs. Energy transfer in vivo is primarily monitored by measuring fluorescence signals from the small fraction of excitations that fail to result in charge separation. Here, we use two-dimensional electronic spectroscopy to follow the entire energy transfer process in a thriving culture of the purple bacteria, Rhodobacter sphaeroides. By removing contributions from scattered light, we extract the dynamics of energy transfer through the dense network of antenna complexes and into the reaction center. Simulations demonstrate that these dynamics constrain the membrane organization into small pools of core antenna complexes that rapidly trap energy absorbed by surrounding peripheral antenna complexes. The rapid trapping and limited back transfer of these excitations lead to transfer efficiencies of 83% and a small functional light-harvesting unit.During photosynthesis, energy is transferred from photosynthetic antenna to reaction centers via ultrafast energy transfer. Here the authors track energy transfer in photosynthetic bacteria using two-dimensional electronic spectroscopy and show that these transfer dynamics constrain antenna complex organization.

  View details for DOI 10.1038/s41467-017-01124-z

  View details for Web of Science ID 000413118000006

  View details for PubMedID 29042567

  View details for PubMedCentralID PMC5715167

• Communication: Broad manifold of excitonic states in light-harvesting complex 1 promotes efficient unidirectional energy transfer in vivo JOURNAL OF CHEMICAL PHYSICS Sohail, S. H., Dahlberg, P. D., Allodi, M. A., Massey, S. C., Ting, P., Martin, E. C., Hunter, C., Engel, G. S. 2017; 147 (13): 131101

  Abstract

  In photosynthetic organisms, the pigment-protein complexes that comprise the light-harvesting antenna exhibit complex electronic structures and ultrafast dynamics due to the coupling among the chromophores. Here, we present absorptive two-dimensional (2D) electronic spectra from living cultures of the purple bacterium, Rhodobacter sphaeroides, acquired using gradient assisted photon echo spectroscopy. Diagonal slices through the 2D lineshape of the LH1 stimulated emission/ground state bleach feature reveal a resolvable higher energy population within the B875 manifold. The waiting time evolution of diagonal, horizontal, and vertical slices through the 2D lineshape shows a sub-100 fs intra-complex relaxation as this higher energy population red shifts. The absorption (855 nm) of this higher lying sub-population of B875 before it has red shifted optimizes spectral overlap between the LH1 B875 band and the B850 band of LH2. Access to an energetically broad distribution of excitonic states within B875 offers a mechanism for efficient energy transfer from LH2 to LH1 during photosynthesis while limiting back transfer. Two-dimensional lineshapes reveal a rapid decay in the ground-state bleach/stimulated emission of B875. This signal, identified as a decrease in the dipole strength of a strong transition in LH1 on the red side of the B875 band, is assigned to the rapid localization of an initially delocalized exciton state, a dephasing process that frustrates back transfer from LH1 to LH2.

  View details for DOI 10.1063/1.4999057

  View details for Web of Science ID 000412321600001

  View details for PubMedID 28987085

  View details for PubMedCentralID PMC5848712

• Charge Separation Related to Photocatalytic H-2 Production from a Ru-Apoflavodoxin-Ni Biohybrid ACS ENERGY LETTERS Soltau, S. R., Niklas, J., Dahlberg, P. D., Mulfort, K. L., Poluektov, O. G., Utschig, L. M. 2017; 2 (1): 230-237

  View details for DOI 10.1021/acsenergylett.6b00614

  View details for Web of Science ID 000392260400032

• Optical Resonance Imaging: An Optical Analog to MRI with Subdiffraction-Limited Capabilities ACS PHOTONICS Allodi, M. A., Dahlberg, P. D., Mazuski, R. J., Davis, H. C., Otto, J. P., Engel, G. S. 2016; 3 (12): 2445-2452

  Abstract

  We propose here optical resonance imaging (ORI), a direct optical analog to magnetic resonance imaging (MRI). The proposed pulse sequence for ORI maps space to time and recovers an image from a heterodyne-detected third-order nonlinear photon echo measurement. As opposed to traditional photon echo measurements, the third pulse in the ORI pulse sequence has significant pulse-front tilt that acts as a temporal gradient. This gradient couples space to time by stimulating the emission of a photon echo signal from different lateral spatial locations of a sample at different times, providing a widefield ultrafast microscopy. We circumvent the diffraction limit of the optics by mapping the lateral spatial coordinate of the sample with the emission time of the signal, which can be measured to high precision using interferometric heterodyne detection. This technique is thus an optical analog of MRI, where magnetic-field gradients are used to localize the spin-echo emission to a point below the diffraction limit of the radio-frequency wave used. We calculate the expected ORI signal using 15 fs pulses and 87° of pulse-front tilt, collected using f/2 optics and find a two-point resolution 275 nm using 800 nm light that satisfies the Rayleigh criterion. We also derive a general equation for resolution in optical resonance imaging that indicates that there is a possibility of superresolution imaging using this technique. The photon echo sequence also enables spectroscopic determination of the input and output energy. The technique thus correlates the input energy with the final position and energy of the exciton.

  View details for DOI 10.1021/acsphotonics.6b00694

  View details for Web of Science ID 000390731700035

  View details for PubMedCentralID PMC5403159

• A simple approach to spectrally resolved fluorescence and bright field microscopy over select regions of interest Dahlberg, P. D., Boughter, C. T., Faruk, N. F., Hong, L., Koh, Y., Reyer, M. A., Shaiber, A., Sherani, A., Zhang, J., Jureller, J. E., Hammond, A. T. AMER INST PHYSICS. 2016: 113704

  Abstract

  A standard wide field inverted microscope was converted to a spatially selective spectrally resolved microscope through the addition of a polarizing beam splitter, a pair of polarizers, an amplitude-mode liquid crystal-spatial light modulator, and a USB spectrometer. The instrument is capable of simultaneously imaging and acquiring spectra over user defined regions of interest. The microscope can also be operated in a bright-field mode to acquire absorption spectra of micron scale objects. The utility of the instrument is demonstrated on three different samples. First, the instrument is used to resolve three differently labeled fluorescent beads in vitro. Second, the instrument is used to recover time dependent bleaching dynamics that have distinct spectral changes in the cyanobacteria, Synechococcus leopoliensis UTEX 625. Lastly, the technique is used to acquire the absorption spectra of CH3NH3PbBr3 perovskites and measure differences between nanocrystal films and micron scale crystals.

  View details for DOI 10.1063/1.4967274

  View details for Web of Science ID 000390242300344

  View details for PubMedID 27910631

  View details for PubMedCentralID PMC5135713

• Electronic Structure and Dynamics of Higher-Lying Excited States in Light Harvesting Complex 1 from Rhodobacter sphaeroides JOURNAL OF PHYSICAL CHEMISTRY A Dahlberg, P. D., Ting, P., Massey, S. C., Martin, E. C., Hunter, C., Engel, G. S. 2016; 120 (24): 4124–30

  Abstract

  Light harvesting in photosynthetic organisms involves efficient transfer of energy from peripheral antenna complexes to core antenna complexes, and ultimately to the reaction center where charge separation drives downstream photosynthetic processes. Antenna complexes contain many strongly coupled chromophores, which complicates analysis of their electronic structure. Two-dimensional electronic spectroscopy (2DES) provides information on energetic coupling and ultrafast energy transfer dynamics, making the technique well suited for the study of photosynthetic antennae. Here, we present 2DES results on excited state properties and dynamics of a core antenna complex, light harvesting complex 1 (LH1), embedded in the photosynthetic membrane of Rhodobacter sphaeroides. The experiment reveals weakly allowed higher-lying excited states in LH1 at 770 nm, which transfer energy to the strongly allowed states at 875 nm with a lifetime of 40 fs. The presence of higher-lying excited states is in agreement with effective Hamiltonians constructed using parameters from crystal structures and atomic force microscopy (AFM) studies. The energy transfer dynamics between the higher- and lower-lying excited states agree with Redfield theory calculations.

  View details for DOI 10.1021/acs.jpca.6b04146

  View details for Web of Science ID 000378663200005

  View details for PubMedID 27232937

  View details for PubMedCentralID PMC5668141

• Mutations to R. sphaeroides Reaction Center Perturb Energy Levels and Vibronic Coupling but Not Observed Energy Transfer Rates JOURNAL OF PHYSICAL CHEMISTRY A Flanagan, M. L., Long, P. D., Dahlberg, P. D., Rolczynski, B. S., Massey, S. C., Engel, G. S. 2016; 120 (9): 1479–87

  Abstract

  The bacterial reaction center is capable of both efficiently collecting and quickly transferring energy within the complex; therefore, the reaction center serves as a convenient model for both energy transfer and charge separation. To spectroscopically probe the interactions between the electronic excited states on the chromophores and their intricate relationship with vibrational motions in their environment, we examine coherences between the excited states. Here, we investigate this question by introducing a series of point mutations within 12 Å of the special pair of bacteriochlorophylls in the Rhodobacter sphaeroides reaction center. Using two-dimensional spectroscopy, we find that the time scales of energy transfer dynamics remain unperturbed by these mutations. However, within these spectra, we detect changes in the mixed vibrational-electronic coherences in these reaction centers. Our results indicate that resonance between bacteriochlorophyll vibrational modes and excitonic energy gaps promote electronic coherences and support current vibronic models of photosynthetic energy transfer.

  View details for DOI 10.1021/acs.jpca.5b08366

  View details for Web of Science ID 000372042200016

  View details for PubMedID 26630123

  View details for PubMedCentralID PMC4824194

• Netrin-1-Regulated Distribution of UNC5B and DCC in Live Cells Revealed by TICCS BIOPHYSICAL JOURNAL Gopal, A. A., Rappaz, B., Rouger, V., Martyn, I. B., Dahlberg, P. D., Meland, R. J., Beamishd, I. V., Kennedy, T. E., Wisemant, P. W. 2016; 110 (3): 623–34

  Abstract

  Netrins are secreted proteins that direct cell migration and adhesion during development. Netrin-1 binds its receptors deleted in colorectal cancer (DCC) and the UNC5 homologs (UNC5A-D) to activate downstream signaling that ultimately directs cytoskeletal reorganization. To investigate how netrin-1 regulates the dynamic distribution of DCC and UNC5 homologs, we applied fluorescence confocal and total internal reflection fluorescence microscopy, and sliding window temporal image cross correlation spectroscopy, to measure time profiles of the plasma membrane distribution, aggregation state, and interaction fractions of fluorescently tagged netrin receptors expressed in HEK293T cells. Our measurements reveal changes in receptor aggregation that are consistent with netrin-1-induced recruitment of DCC-enhanced green fluorescent protein (EGFP) from intracellular vesicles to the plasma membrane. Netrin-1 also induced colocalization of coexpressed full-length DCC-EGFP with DCC-T-mCherry, a putative DCC dominant negative that replaces the DCC intracellular domain with mCherry, consistent with netrin-1-induced receptor oligomerization, but with no change in aggregation state with time, providing evidence that signaling via the DCC intracellular domain triggers DCC recruitment to the plasma membrane. UNC5B expressed alone was also recruited by netrin-1 to the plasma membrane. Coexpressed DCC and UNC5 homologs are proposed to form a heteromeric netrin-receptor complex to mediate a chemorepellent response. Application of temporal image cross correlation spectroscopy to image series of cells coexpressing UNC5B-mCherry and DCC-EGFP revealed a netrin-1-induced increase in colocalization, with both receptors recruited to the plasma membrane from preexisting clusters, consistent with vesicular recruitment and receptor heterooligomerization. Plasma membrane recruitment of DCC or UNC5B was blocked by application of the netrin-1 VI-V peptide, which fails to activate chemoattraction, or by pharmacological block of Src family kinase signaling, consistent with receptor recruitment requiring netrin-1-activated signaling. Our findings reveal a mechanism activated by netrin-1 that recruits DCC and UNC5B to the plasma membrane.

  View details for DOI 10.1016/j.bpj.2015.12.022

  View details for Web of Science ID 000369467800012

  View details for PubMedID 26840727

  View details for PubMedCentralID PMC4744167

• Electronic and nuclear contributions to time-resolved optical and X-ray absorption spectra of hematite and insights into photoelectrochemical performance Energy & Environmental Science Hayes, D., Hadt, R. G., Emery, J. D., Cordones, A. A., Martinson, A. F., Shelby, M. L., Fransted, K. A., Dahlberg, P. D., Hong, J., Zhang, X., Kong, Q., Schoenlein, R. W., Chen, L. X. 2016; 9 (12): 3754–69

  View details for DOI 10.1039/c6ee02266a

  View details for Web of Science ID 000392915500016

• Ru-protein-Co biohybrids designed for solar hydrogen production: understanding electron transfer pathways related to photocatalytic function CHEMICAL SCIENCE Soltau, S. R., Dahlberg, P. D., Niklas, J., Poluektov, O. G., Mulfort, K. L., Utschig, L. M. 2016; 7 (12): 7068–78

  Abstract

  A series of Ru-protein-Co biohybrids have been prepared using the electron transfer proteins ferredoxin (Fd) and flavodoxin (Fld) as scaffolds for photocatalytic hydrogen production. The light-generated charge separation within these hybrids has been monitored by transient optical and electron paramagnetic resonance spectroscopies. Two distinct electron transfer pathways are observed. The Ru-Fd-Co biohybrid produces up to 650 turnovers of H2 utilizing an oxidative quenching mechanism for Ru(ii)* and a sequential electron transfer pathway via the native [2Fe-2S] cluster to generate a Ru(iii)-Fd-Co(i) charge separated state that lasts for ∼6 ms. In contrast, a direct electron transfer pathway occurs for the Ru-ApoFld-Co biohybrid, which lacks an internal electron relay, generating Ru(i)-ApoFld-Co(i) charge separated state that persists for ∼800 μs and produces 85 turnovers of H2 by a reductive quenching mechanism for Ru(ii)*. This work demonstrates the utility of protein architectures for linking donor and catalytic function via direct or sequential electron transfer pathways to enable stabilized charge separation which facilitates photocatalysis for solar fuel production.

  View details for DOI 10.1039/c6sc03121h

  View details for Web of Science ID 000391453200028

  View details for PubMedID 28451142

  View details for PubMedCentralID PMC5355951

• Red, Yellow, Green, and Blue Amplified Spontaneous Emission and Lasing Using Colloidal CdSe Nanoplatelets ACS NANO She, C., Fedin, I., Dolzhnikov, D. S., Dahlberg, P. D., Engel, G. S., Schaller, R. D., Talapin, D. V. 2015; 9 (10): 9475–85

  Abstract

  There have been multiple demonstrations of amplified spontaneous emission (ASE) and lasing using colloidal semiconductor nanocrystals. However, it has been proven difficult to achieve low thresholds suitable for practical use of nanocrystals as gain media. Low-threshold blue ASE and lasing from nanocrystals is an even more challenging task. Here, we show that colloidal nanoplatelets (NPLs) with electronic structure of quantum wells can produce ASE in the red, yellow, green, and blue regions of the visible spectrum with low thresholds and high gains. In particular, for blue-emitting NPLs, the ASE threshold is 50 μJ/cm(2), lower than any reported value for nanocrystals. We then demonstrate red, yellow, green, and blue lasing using NPLs with different thicknesses. We find that the lateral size of NPLs does not show any strong effect on the Auger recombination rates and, correspondingly, on the ASE threshold or gain saturation. This observation highlights the qualitative difference of multiexciton dynamics in CdSe NPLs and other quantum-confined CdSe materials, such as quantum dots and rods. Our measurements of the gain bandwidth and gain lifetime further support the prospects of colloidal NPLs as solution-processed optical gain materials.

  View details for DOI 10.1021/acsnano.5b02509

  View details for Web of Science ID 000363915300006

  View details for PubMedID 26302368

• Communication: Coherences observed in vivo in photosynthetic bacteria using two-dimensional electronic spectroscopy JOURNAL OF CHEMICAL PHYSICS Dahlberg, P. D., Norris, G. J., Wang, C., Viswanathan, S., Singh, V. P., Engel, G. S. 2015; 143 (10): 101101

  Abstract

  Energy transfer through large disordered antenna networks in photosynthetic organisms can occur with a quantum efficiency of nearly 100%. This energy transfer is facilitated by the electronic structure of the photosynthetic antennae as well as interactions between electronic states and the surrounding environment. Coherences in time-domain spectroscopy provide a fine probe of how a system interacts with its surroundings. In two-dimensional electronic spectroscopy, coherences can appear on both the ground and excited state surfaces revealing detailed information regarding electronic structure, system-bath coupling, energy transfer, and energetic coupling in complex chemical systems. Numerous studies have revealed coherences in isolated photosynthetic pigment-protein complexes, but these coherences have not been observed in vivo due to the small amplitude of these signals and the intense scatter from whole cells. Here, we present data acquired using ultrafast video-acquisition gradient-assisted photon echo spectroscopy to observe quantum beating signals from coherences in vivo. Experiments were conducted on isolated light harvesting complex II (LH2) from Rhodobacter sphaeroides, whole cells of R. sphaeroides, and whole cells of R. sphaeroides grown in 30% deuterated media. A vibronic coherence was observed following laser excitation at ambient temperature between the B850 and the B850(∗) states of LH2 in each of the 3 samples with a lifetime of ∼40-60 fs.

  View details for DOI 10.1063/1.4930539

  View details for Web of Science ID 000361572900001

  View details for PubMedID 26373989

  View details for PubMedCentralID PMC4567573

• Towards quantification of vibronic coupling in photosynthetic antenna complexes JOURNAL OF CHEMICAL PHYSICS Singh, V. P., Westberg, M., Wang, C., Dahlberg, P. D., Gellen, T., Gardiner, A. T., Cogdell, R. J., Engel, G. S. 2015; 142 (21): 212446

  Abstract

  Photosynthetic antenna complexes harvest sunlight and efficiently transport energy to the reaction center where charge separation powers biochemical energy storage. The discovery of existence of long lived quantum coherence during energy transfer has sparked the discussion on the role of quantum coherence on the energy transfer efficiency. Early works assigned observed coherences to electronic states, and theoretical studies showed that electronic coherences could affect energy transfer efficiency--by either enhancing or suppressing transfer. However, the nature of coherences has been fiercely debated as coherences only report the energy gap between the states that generate coherence signals. Recent works have suggested that either the coherences observed in photosynthetic antenna complexes arise from vibrational wave packets on the ground state or, alternatively, coherences arise from mixed electronic and vibrational states. Understanding origin of coherences is important for designing molecules for efficient light harvesting. Here, we give a direct experimental observation from a mutant of LH2, which does not have B800 chromophores, to distinguish between electronic, vibrational, and vibronic coherence. We also present a minimal theoretical model to characterize the coherences both in the two limiting cases of purely vibrational and purely electronic coherence as well as in the intermediate, vibronic regime.

  View details for DOI 10.1063/1.4921324

  View details for Web of Science ID 000355931800050

  View details for PubMedID 26049466

  View details for PubMedCentralID PMC4441712

• Aqueous light driven hydrogen production by a Ru-ferredoxin-Co biohybrid CHEMICAL COMMUNICATIONS Soltau, S. R., Niklas, J., Dahlberg, P. D., Poluektov, O. G., Tiede, D. M., Mulfort, K. L., Utschig, L. M. 2015; 51 (53): 10628–31

  Abstract

  Herein we report the creation of a novel solar fuel biohybrid for light-driven H2 production utilizing the native electron transfer protein ferredoxin (Fd) as a scaffold for binding of a ruthenium photosensitizer (PS) and a molecular cobaloxime catalyst (Co). EPR and transient optical experiments provide direct evidence of a long-lived (>1.5 ms) Ru(III)-Fd-Co(I) charge separated state formed via an electron relay through the Fd [2Fe-2S] cluster, initiating the catalytic cycle for 2H(+) + 2e(-) → H2.

  View details for DOI 10.1039/c5cc03006d

  View details for Web of Science ID 000356453200008

  View details for PubMedID 26051070

• Dispersion-free continuum two-dimensional electronic spectrometer APPLIED OPTICS Zheng, H., Caram, J. R., Dahlberg, P. D., Rolczynski, B. S., Viswanathan, S., Dolzhnikov, D. S., Khadivi, A., Talapin, D. V., Engel, G. S. 2014; 53 (9): 1909–17

  Abstract

  Electronic dynamics span broad energy scales with ultrafast time constants in the condensed phase. Two-dimensional (2D) electronic spectroscopy permits the study of these dynamics with simultaneous resolution in both frequency and time. In practice, this technique is sensitive to changes in nonlinear dispersion in the laser pulses as time delays are varied during the experiment. We have developed a 2D spectrometer that uses broadband continuum generated in argon as the light source. Using this visible light in phase-sensitive optical experiments presents new challenges in implementation. We demonstrate all-reflective interferometric delays using angled stages. Upon selecting an ~180  nm window of the available bandwidth at ~10  fs compression, we probe the nonlinear response of broadly absorbing CdSe quantum dots and electronic transitions of Chlorophyll a.

  View details for DOI 10.1364/AO.53.001909

  View details for Web of Science ID 000333343900025

  View details for PubMedID 24663470

  View details for PubMedCentralID PMC4349747

• Exploring size and state dynamics in CdSe quantum dots using two-dimensional electronic spectroscopy JOURNAL OF CHEMICAL PHYSICS Caram, J. R., Zheng, H., Dahlberg, P. D., Rolczynski, B. S., Griffin, G. B., Dolzhnikov, D. S., Talapin, D. V., Engel, G. S. 2014; 140 (8): 084701

  Abstract

  Development of optoelectronic technologies based on quantum dots depends on measuring, optimizing, and ultimately predicting charge carrier dynamics in the nanocrystal. In such systems, size inhomogeneity and the photoexcited population distribution among various excitonic states have distinct effects on electron and hole relaxation, which are difficult to distinguish spectroscopically. Two-dimensional electronic spectroscopy can help to untangle these effects by resolving excitation energy and subsequent nonlinear response in a single experiment. Using a filament-generated continuum as a pump and probe source, we collect two-dimensional spectra with sufficient spectral bandwidth to follow dynamics upon excitation of the lowest three optical transitions in a polydisperse ensemble of colloidal CdSe quantum dots. We first compare to prior transient absorption studies to confirm excitation-state-dependent dynamics such as increased surface-trapping upon excitation of hot electrons. Second, we demonstrate fast band-edge electron-hole pair solvation by ligand and phonon modes, as the ensemble relaxes to the photoluminescent state on a sub-picosecond time-scale. Third, we find that static disorder due to size polydispersity dominates the nonlinear response upon excitation into the hot electron manifold; this broadening mechanism stands in contrast to that of the band-edge exciton. Finally, we demonstrate excitation-energy dependent hot-carrier relaxation rates, and we describe how two-dimensional electronic spectroscopy can complement other transient nonlinear techniques.

  View details for DOI 10.1063/1.4865832

  View details for Web of Science ID 000332485900046

  View details for PubMedID 24588185

  View details for PubMedCentralID PMC3977796

• Dynamic localization of electronic excitation in photosynthetic complexes revealed with chiral two-dimensional spectroscopy NATURE COMMUNICATIONS Fidler, A. F., Singh, V. P., Long, P. D., Dahlberg, P. D., Engel, G. S. 2014; 5: 3286

  Abstract

  Time-resolved ultrafast optical probes of chiral dynamics provide a new window allowing us to explore how interactions with such structured environments drive electronic dynamics. Incorporating optical activity into time-resolved spectroscopies has proven challenging because of the small signal and large achiral background. Here we demonstrate that two-dimensional electronic spectroscopy can be adapted to detect chiral signals and that these signals reveal how excitations delocalize and contract following excitation. We dynamically probe the evolution of chiral electronic structure in the light-harvesting complex 2 of purple bacteria following photoexcitation by creating a chiral two-dimensional mapping. The dynamics of the chiral two-dimensional signal directly reports on changes in the degree of delocalization of the excitonic states following photoexcitation. The mechanism of energy transfer in this system may enhance transfer probability because of the coherent coupling among chromophores while suppressing fluorescence that arises from populating delocalized states. This generally applicable spectroscopy will provide an incisive tool to probe ultrafast transient molecular fluctuations that are obscured in non-chiral experiments.

  View details for DOI 10.1038/ncomms4286

  View details for Web of Science ID 000332667600033

  View details for PubMedID 24504144

  View details for PubMedCentralID PMC3976994

• Persistent Interexcitonic Quantum Coherence in CdSe Quantum Dots JOURNAL OF PHYSICAL CHEMISTRY LETTERS Caram, J. R., Zheng, H., Dahlberg, P. D., Rolczynski, B. S., Griffin, G. B., Fidler, A. F., Dolzhnikov, D. S., Talapin, D. V., Engel, G. S. 2014; 5 (1): 196–204

  Abstract

  The creation and manipulation of quantum superpositions is a fundamental goal for the development of materials with novel optoelectronic properties. In this letter, we report persistent (~80 fs lifetime) quantum coherence between the 1S and 1P excitonic states in zinc-blende colloidal CdSe quantum dots at room temperature, measured using Two-Dimensional Electronic Spectroscopy. We demonstrate that this quantum coherence manifests as an intradot phenomenon, the frequency of which depends on the size of the dot excited within the ensemble of QDs. We model the lifetime of the coherence and demonstrate that correlated interexcitonic fluctuations preserve relative phase between excitonic states. These observations suggest an avenue for engineering long-lived interexcitonic quantum coherence in colloidal quantum dots.

  View details for DOI 10.1021/jz402336t

  View details for Web of Science ID 000329331400032

  View details for PubMedID 24719679

  View details for PubMedCentralID PMC3976995

• Energy Transfer Observed in Live Cells Using Two-Dimensional Electronic Spectroscopy JOURNAL OF PHYSICAL CHEMISTRY LETTERS Dahlberg, P. D., Fidler, A. F., Caram, J. R., Long, P. D., Engel, G. S. 2013; 4 (21): 3636–40

  Abstract

  Two-dimensional electronic spectroscopy (2DES) elucidates electronic structure and dynamics on a femtosecond time scale and has proven to be an incisive tool for probing congested linear spectra of biological systems. However, samples that scatter light intensely frustrate 2DES analysis, necessitating the use of isolated protein chromophore complexes when studying photosynthetic energy transfer processes. We present a method for conducting 2DES experiments that takes only seconds to acquire thousands of 2DES spectra and permits analysis of highly scattering samples, specifically whole cells of the purple bacterium Rhodobacter sphaeroides. These in vivo 2DES experiments reveal similar timescales for energy transfer within the antennae complex (light harvesting complex 2, LH2) both in the native photosynthetic membrane environment and in isolated detergent micelles.

  View details for DOI 10.1021/jz401944q

  View details for Web of Science ID 000326845200009

  View details for PubMedID 24478821

  View details for PubMedCentralID PMC3902138

• Probing energy transfer events in the light harvesting complex 2 (LH2) of Rhodobacter sphaeroides with two-dimensional spectroscopy JOURNAL OF CHEMICAL PHYSICS Fidler, A. F., Singh, V. P., Long, P. D., Dahlberg, P. D., Engel, G. S. 2013; 139 (15): 155101

  Abstract

  Excitation energy transfer events in the photosynthetic light harvesting complex 2 (LH2) of Rhodobacter sphaeroides are investigated with polarization controlled two-dimensional electronic spectroscopy. A spectrally broadened pulse allows simultaneous measurement of the energy transfer within and between the two absorption bands at 800 nm and 850 nm. The phased all-parallel polarization two-dimensional spectra resolve the initial events of energy transfer by separating the intra-band and inter-band relaxation processes across the two-dimensional map. The internal dynamics of the 800 nm region of the spectra are resolved as a cross peak that grows in on an ultrafast time scale, reflecting energy transfer between higher lying excitations of the B850 chromophores into the B800 states. We utilize a polarization sequence designed to highlight the initial excited state dynamics which uncovers an ultrafast transfer component between the two bands that was not observed in the all-parallel polarization data. We attribute the ultrafast transfer component to energy transfer from higher energy exciton states to lower energy states of the strongly coupled B850 chromophores. Connecting the spectroscopic signature to the molecular structure, we reveal multiple relaxation pathways including a cyclic transfer of energy between the two rings of the complex.

  View details for DOI 10.1063/1.4824637

  View details for Web of Science ID 000326116600056

  View details for PubMedID 24160544

  View details for PubMedCentralID PMC3815049

• Time Scales of Coherent Dynamics in the Light-Harvesting Complex 2 (LH2) of Rhodobacter sphaeroides JOURNAL OF PHYSICAL CHEMISTRY LETTERS Fidler, A. F., Singh, V. P., Long, P. D., Dahlberg, P. D., Engel, G. S. 2013; 4 (9): 1404–9

  Abstract

  The initial dynamics of energy transfer in the light harvesting complex 2 from Rhodobacter sphaeroides were investigated with polarization controlled two-dimensional spectroscopy. This method allows only the coherent electronic motions to be observed revealing the timescale of dephasing among the excited states. We observe persistent coherence among all states and assign ensemble dephasing rates for the various coherences. A simple model is utilized to connect the spectroscopic transitions to the molecular structure, allowing us to distinguish coherences between the two rings of chromophores and coherences within the rings. We also compare dephasing rates between excited states to dephasing rates between the ground and excited states, revealing that the coherences between excited states dephase on a slower timescale than coherences between the ground and excited states.

  View details for DOI 10.1021/jz400438m

  View details for Web of Science ID 000318536500007

  View details for PubMedID 23878622

  View details for PubMedCentralID PMC3714110