Ultrafast primary processes in photosystem I of the cyanobacterium Synechocystis sp PCC 6803

Ultrafast primary processes in the trimeric photosystem I core antenna-reac tion center complex of the cyanobacterium Synechocystis sp. PCC 6803 have b een examined in pump-probe experiments with similar to 100 fs resolution. A global analysis of two-color profiles, excited at 660 nm and probed at 5 n m intervals from 650 to 730 nm, reveals 430 fs kinetics for spectral equili bration among bulk antenna chlorophylls, At least two lifetime components ( 2.0 and 6.5 ps in our analysis) are required to describe equilibration of b ulk chlorophylls with far red-absorbing chlorophylls (>700 nm), Trapping at P700 occurs with 24-ps kinetics. The multiphasic bulk <-> red equilibratio n kinetics are intriguing, because prior steady-state spectral studies have suggested that the core antenna in Synechocystis sp. contains only one red -absorbing chlorophyll species (C708). The disperse kinetics may arise from inhomogeneous broadening in C708, The one-color optical anisotropy at 680 nm (near the red edge of the bulk antenna) decays with 590 fs kinetics; the corresponding anisotropy at 710 nm shows similar to 3.1 ps kinetics. The l atter may signal equilibration among symmetry-equivalent red chlorophylls, bound to different monomers within trimeric photosystem I.