Subpicosecond Transient Absorption Difference Spectroscopy on the Reaction Center of Photosystem II: Radical Pair Formation at 77 K

To further characterize the processes of charge separation and excitation energy transfer in the isolated photosystem II reaction center complex (the D1-D2-cytochrome b-559 complex), subpicosecond absorption difference spectra were measured at 77 K. The preparations were excited with a repetition rate of 30 Hz by laser pulses with a duration of 300-400 fs at 688, 681, 672, and 670.5 nm. Upon excitation at 688 and 681 nm, a biphasic decay of the induced signals with time constants of 1-2 ps and 80-100 ps was observed. The 1-2 ps component is interpreted as the intrinsic charge separation time from the singlet-excited state of P680, the primary electron donor of photosystem II. The 80-100 ps time is interpreted as charge separation limited by slow energy transfer from a trap state in the photosystem II reaction center degenerate with P680, earlier identified by low-temperature fluorescence experiments. In addition, upon excitation at 672 and 670.5 nm energetically downhill energy transfer processes of 400-500 fs and nearly 14 ps were observed, which according to the associated spectral changes, originate from states in the reaction center absorbing at different energies. 40 refs., 6 figs.