@article{17d2e0f8ca934fda85783356d57e1a00,
title = "Local protein solvation drives direct down-conversion in phycobiliprotein PC645 via incoherent vibronic transport",
abstract = "The mechanisms controlling excitation energy transport (EET) in light-harvesting complexes remain controversial. Following the observation of long-lived beats in 2D electronic spectroscopy of PC645, vibronic coherence, the delocalization of excited states between pigments supported by a resonant vibration, has been proposed to enable direct excitation transport from the highest-energy to the lowest-energy pigments, bypassing a collection of intermediate states. Here, we instead show that for phycobiliprotein PC645 an incoherent vibronic transport mechanism is at play. We quantify the solvation dynamics of individual pigments using ab initio quantum mechanics/molecular mechanics (QM/MM) nuclear dynamics. Our atomistic spectral densities reproduce experimental observations ranging from absorption and fluorescence spectra to the timescales and selectivity of down-conversion observed in transient absorption measurements. We construct a general model for vibronic dimers and establish the parameter regimes of coherent and incoherent vibronic transport. We demonstrate that direct down-conversion in PC645 proceeds incoherently, enhanced by large reorganization energies and a broad collection of high-frequency vibrations. We suggest that a similar incoherent mechanism is appropriate across phycobiliproteins and represents a potential design principle for nanoscale control of EET.",
keywords = "Excitation energy transfer, Light harvesting, Molecular dynamics, Photosynthesis, Quantum coherence",
author = "Blau, {Samuel M.} and Bennett, {Doran I.G.} and Christoph Kreisbeck and Scholes, {Gregory D.} and Al{\'a}n Aspuru-Guzik",
note = "Funding Information: We gratefully acknowledge detailed discussions with and extensive commentary on the manuscript by Kapil Amarnath, as well as helpful conversations with Jacob Dean. We thank Thomas Markovich for help obtaining correlation functions and spectral densities. We acknowledge support from the Center for Excitonics, an Energy Frontier Research Center funded by the US Department of Energy (DOE), Office of Science and Office of Basic Energy Sciences, under Award DE-SC0001088. S.M.B. acknowledges support from the US DOE through the Computational Sciences Graduate Fellowship. D.I.G.B. and A.A.-G. acknowledge the John Templeton Foundation (Grant 60469). D.I.G.B., A.A.-G., and G.D.S. acknowledge the Canadian Institute for Advanced Research for support through the Bio-Inspired Solar Energy program. This research used resources of the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the Office of Science of the US DOE under Contract DE-AC02-05CH11231. We thank Nvidia for support via the Harvard CUDA Center of Excellence. This research used computational time on the Odyssey cluster, supported by the Faculty of Arts and Sciences Research Computing Group at Harvard University. Funding Information: ACKNOWLEDGMENTS. We gratefully acknowledge detailed discussions with and extensive commentary on the manuscript by Kapil Amarnath, as well as helpful conversations with Jacob Dean. We thank Thomas Markovich for help obtaining correlation functions and spectral densities. We acknowledge support from the Center for Excitonics, an Energy Frontier Research Center funded by the US Department of Energy (DOE), Office of Science and Office of Basic Energy Sciences, under Award DE-SC0001088. S.M.B. acknowledges support from the US DOE through the Computational Sciences Graduate Fellowship. D.I.G.B. and A.A.-G. acknowledge the John Templeton Foundation (Grant 60469). D.I.G.B., A.A.-G., and G.D.S. acknowledge the Canadian Institute for Advanced Research for support through the Bio-Inspired Solar Energy program. This research used resources of the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the Office of Science of the US DOE under Contract DE-AC02-05CH11231. We thank Nvidia for support via the Harvard CUDA Center of Excellence. This research used computational time on the Odyssey cluster, supported by the Faculty of Arts and Sciences Research Computing Group at Harvard University. Publisher Copyright: {\textcopyright} 2018 National Academy of Sciences. All Rights Reserved.",
year = "2018",
doi = "10.1073/pnas.1800370115",
language = "English (US)",
volume = "115",
pages = "E3342--E3350",
journal = "Proceedings of the National Academy of Sciences of the United States of America",
issn = "0027-8424",
publisher = "National Academy of Sciences",
number = "15",
}