Effect of Dehydrated Trehalose Matrix on the Kinetics of Forward Electron Transfer Reactions in Photosystem i

The effect of dehydration on the kinetics of forward electron transfer (ET) has been studied in cyanobacterial photosystem I (PS I) complexes in a trehalose glassy matrix by time-resolved optical and EPR spectroscopies in the 100 fs to 1 ms time domain. The kinetics of the flash-induced absorption changes in the subnanosecond time domain due to primary and secondary charge separation steps were monitored by pump-probe laser spectroscopy with 20-fs lowenergy pump pulses centered at 720 nm. The back-reaction kinetics of P₇₀₀ were measured by high-field time-resolved EPR spectroscopy and the forward kinetics of A_1A•-/A_1B•-F_x by time-resolved optical spectroscopy at 480 nm. The kinetics of the primary ET reactions to form the primary P^•+₇₀₀ A₀•-and the secondary P^•+₇₀₀ A₀•-ion radical pairs were not affected by dehydration in the trehalose matrix, while the yield of the P^•+₇₀₀ A₁. was decreased by ~20%. Forward ET from the phylloquinone molecules in the A_1A•. and A_1B. sites to the iron-sulfur cluster F_X slowed from ~ 220 ns and ~ 20 ns in solution to ~ 13 μs and ~ 80 ns, respectively. However, as shown by EPR spectroscopy, the ~ 15 μs kinetic phase also contains a small contribution from the recombination between A_1B•. and P^•+₇₀₀. These data reveal that the initial ET reactions from P₇₀₀ to secondary phylloquinone acceptors in the A-and B-branches of cofactors (A_1A and A_1B) remain unaffected whereas ET beyond A_1A and A_1B is slowed or prevented by constrained protein dynamics due to the dry trehalose glass matrix.