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 highestenergy 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.

Authors:

Blau, Samuel M. ^[1]; Bennett, Doran I. G. ^[2]; Kreisbeck, Christoph ^[1]; Scholes, Gregory D. ^[3]; Aspuru-Guzik, Alán ^[2]

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+ Show Author Affiliations

1. Department of Chemistry and Chemical Biology, Harvard University, Cambridge, MA 02138,
2. Department of Chemistry and Chemical Biology, Harvard University, Cambridge, MA 02138,, Bio-Inspired Solar Energy Program, Canadian Institute for Advanced Research, Toronto, ON M5G 1Z8, Canada,
3. Bio-Inspired Solar Energy Program, Canadian Institute for Advanced Research, Toronto, ON M5G 1Z8, Canada,, Department of Chemistry, Princeton University, Princeton, NJ 08544

Publication Date:

Tue Mar 27 00:00:00 EDT 2018

Research Org.:

Energy Frontier Research Centers (EFRC) (United States). Center for Excitonics (CE)

Sponsoring Org.:

USDOE Office of Science (SC), Basic Energy Sciences (BES); John Templeton Foundation

OSTI Identifier:

1429951

Alternate Identifier(s):

OSTI ID: 1470506

Grant/Contract Number:  

SC0001088; AC02- 05CH11231; 60469; AC02-05CH11231

Resource Type:

Published Article

Journal Name:

Proceedings of the National Academy of Sciences of the United States of America

Additional Journal Information:

Journal Name: Proceedings of the National Academy of Sciences of the United States of America Journal Volume: 115 Journal Issue: 15; Journal ID: ISSN 0027-8424

Publisher:

Proceedings of the National Academy of Sciences

Country of Publication:

United States

Language:

English

Subject:

09 BIOMASS FUELS; 59 BASIC BIOLOGICAL SCIENCES; 14 SOLAR ENERGY; 37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; solar (photovoltaic); solid state lighting; photosynthesis (natural and artificial); charge transport; optics; synthesis (novel materials); synthesis (self-assembly); synthesis (scalable processing); light harvesting; excitation energy transfer; quantum coherence; molecular dynamics