Abstract
Photosynthetic water oxidation takes place in the water oxidizing center (WOC) of photosystem II (PSII) through the S-state cycle of five intermediates (S0-S4). Determining the efficiencies of the S-state transitions is essential for clarifying the mechanism of water oxidation. In this study, we have estimated the efficiencies of individual S-state transitions using Fourier transform infrared (FTIR) difference spectroscopy. Flash-induced FTIR difference spectra were measured using PSII core complexes of T. elongatus in the presence of ferricyanide as an electron acceptor. Flash-induced electron transfer from WOC to ferricyanide was detected by the CN stretching bands of ferricyanide/ferrocyanide. The flash-number dependence of the intensities of these signals showed a typical period-four oscillation pattern. Simulation of this pattern provided the efficiencies of 96, 97, 95, and 92% for the S0-to-S1, S1-to-S2, S2-to-S3, and S3-to-S0 transitions, respectively. The tendency of the lowest efficiency in the S3-to-S0 transition was also observed in spinach PSII membranes, indicating that this is a general feature in the water oxidation reaction.
