The Quenching of Long-Wavelength Fluorescence by the Closed Reaction Center in Photosystem I in Thermostichus vulcanus at 77 K

Akhtar, Parveen and van, Stokkum I.H.M. and Lambrev, Petar (2024) The Quenching of Long-Wavelength Fluorescence by the Closed Reaction Center in Photosystem I in Thermostichus vulcanus at 77 K. INTERNATIONAL JOURNAL OF MOLECULAR SCIENCES, 25 (22). No.-12430. ISSN 1661-6596

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Abstract

Photosystem I in most organisms contains long-wavelength or “Red” chlorophylls (Chls) absorbing light beyond 700 nm. At cryogenic temperatures, the Red Chls become quasi-traps for excitations as uphill energy transfer is blocked. One pathway for de-excitation of the Red Chls is via transfer to the oxidized RC (P700+), which has broad absorption in the near-infrared region. This study investigates the excitation dynamics of Red Chls in Photosystem I from the cyanobacterium Thermostichus vulcanus at cryogenic temperatures (77 K) and examines the role of the oxidized RC in modulating their fluorescence kinetics. Using time-resolved fluorescence spectroscopy, the kinetics of Red Chls were recorded for samples with open (neutral P700) and closed (P700+) RCs. We found that emission lifetimes in the range of 710–720 nm remained unaffected by the RC state, while more red-shifted emissions (>730 nm) decayed significantly faster when the RC was closed. A kinetic model describing the quenching by the oxidized RC was constructed based on simultaneous fitting to the recorded fluorescence emission in Photosystem I with open and closed RCs. The analysis resolved multiple Red Chl forms and variable quenching efficiencies correlated with their spectral properties. Only the most red-shifted Chls, with emission beyond 730 nm, are efficiently quenched by P700+, with rate constants of up to 6 ns−1. The modeling results support the notion that structural and energetic disorder in Photosystem I can have a comparable or larger effect on the excitation dynamics than the geometric arrangement of Chls. © 2024 by the authors.

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