Ultrafast Primary Processes in Photosystem I of the Cyanobacterium Synechocystis sp. PCC 6803

AbstractUltrafast primary processes in the trimeric photosystem I core antenna-reaction center complex of the cyanobacterium Synechocystis sp. PCC 6803 have been examined in pump-probe experiments with ∼100fs resolution. A global analysis of two-color profiles, excited at 660nm and probed at 5nm intervals from 650 to 730nm, reveals 430fs kinetics for spectral equilibration among bulk antenna chlorophylls. At least two lifetime components (2.0 and 6.5ps in our analysis) are required to describe equilibration of bulk chlorophylls with far red-absorbing chlorophylls (>700nm). Trapping at P700 occurs with 24-ps kinetics. The multiphasic bulk ↔ red equilibration 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 680nm (near the red edge of the bulk antenna) decays with 590fs kinetics; the corresponding anisotropy at 710nm shows ∼3.1ps kinetics. The latter may signal equilibration among symmetry-equivalent red chlorophylls, bound to different monomers within trimeric photosystem I