Symmetry breaking in photosystem I: ultrafast optical studies of variants near the accessory chlorophylls in the A- and B-branches of electron transfer cofactors

Femtosecond absorption spectroscopy of Photosystem I (PS I) complexes from the cyanobacterium Synechocystis sp. PCC 6803 was carried out on three pairs complementary amino acid substitutions located near second pair chlorophyll molecules Chl2A and Chl2B (also termed A-1A A-1B). The dynamics at delays 0.1–500 ps were analyzed by decomposition into discrete decay-associated spectra continuously distributed exponential components. multi-exponential deconvolution changes revealed that electron transfer reactions in PsaA-N600M, PsaA-N600H, PsaA-N600L variants B-branch cofactors are similar to those wild type, while PsaB-N582M, PsaB-N582H, PsaB-N582L A-branch cause significant alterations photochemical processes, making them heterogeneous poorly described a kinetic model. A redistribution unpaired between third monomers Chl2A/Chl2B Chl3A/Chl3B identified time range 9–20 ps, subsequent reduction A1 24–70 ps. In PsaB-N582H/L variants, occurred with decreased quantum yield charge separation. correlates slowing phylloquinone A0 → reduction, but not initial transient measured shortest delay. results support branch competition model, where is sheared Chl2A–Chl3A Chl2B–Chl3B before its either A1A or A1B sites.