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Two-dimensional electronic spectroscopy of a minimal photosystem I complex reveals the rate of primary charge separation

DOI: 10.1021/jacs.1c05010 DOI Help

Authors: Parveen Akhtar (Nanyang Technological University; ELI-HU Non-profit Ltd) , Ido Caspy (Tel Aviv University) , Paweł J. Nowakowski (Nanyang Technological University) , Tirupathi Malavath (Tel Aviv University; The Hebrew University of Jerusalem) , Nathan Nelson (Tel Aviv University) , Howe-Siang Tan (Nanyang Technological University) , Petar H. Lambrev (Hungarian Academy of Sciences)
Co-authored by industrial partner: No

Type: Journal Paper
Journal: Journal Of The American Chemical Society

State: Published (Approved)
Published: September 2021
Diamond Proposal Number(s): 21643

Abstract: Photosystem I (PSI), found in all oxygenic photosynthetic organisms, uses solar energy to drive electron transport with nearly 100% quantum efficiency, thanks to fast energy transfer among antenna chlorophylls and charge separation in the reaction center. There is no complete consensus regarding the kinetics of the elementary steps involved in the overall trapping, especially the rate of primary charge separation. In this work, we employed two-dimensional coherent electronic spectroscopy to follow the dynamics of energy and electron transfer in a monomeric PSI complex from Synechocystis PCC 6803, containing only subunits A–E, K, and M, at 77 K. We also determined the structure of the complex to 4.3 Å resolution by cryoelectron microscopy with refinements to 2.5 Å. We applied structure-based modeling using a combined Redfield–Förster theory to compute the excitation dynamics. The absorptive 2D electronic spectra revealed fast excitonic/vibronic relaxation on time scales of 50–100 fs from the high-energy side of the absorption spectrum. Antenna excitations were funneled within 1 ps to a small pool of chlorophylls absorbing around 687 nm, thereafter decaying with 4–20 ps lifetimes, independently of excitation wavelength. Redfield–Förster energy transfer computations showed that the kinetics is limited by transfer from these red-shifted pigments. The rate of primary charge separation, upon direct excitation of the reaction center, was determined to be 1.2–1.5 ps–1. This result implies activationless electron transfer in PSI.

Journal Keywords: Antennas; Absorption; Quantum mechanics; Fluorescence resonance energy transfer; Kinetics

Diamond Keywords: Photosynthesis

Subject Areas: Biology and Bio-materials, Chemistry

Diamond Offline Facilities: Electron Bio-Imaging Centre (eBIC)
Instruments: Krios III-Titan Krios III at Diamond

Added On: 06/09/2021 09:11

Discipline Tags:

Physical Chemistry Biochemistry Chemistry Structural biology Life Sciences & Biotech

Technical Tags:

Microscopy Electron Microscopy (EM) Cryo Electron Microscopy (Cryo EM)