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

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 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. 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.