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 A1A•-/A1B•-Fx 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 A1A•. and A1B. sites to the iron-sulfur cluster FX 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 A1B•. 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 (A1A and A1B) remain unaffected whereas ET beyond A1A and A1B is slowed or prevented by constrained protein dynamics due to the dry trehalose glass matrix.
All Science Journal Classification (ASJC) codes
- Physical and Theoretical Chemistry