Abstract
In the water oxidizing center of photosystem II, two water molecules are known to cleave into one molecular oxygen and four protons through a light-driven cycle of the S states (S0-S4). However, the definite conclusion has yet to be reached about the number of protons released during individual S-state transitions. In this study, we have estimated the proton release pattern during water oxidation using Fourier transform infrared (FTIR) spectroscopy. The PSII core complexes from the YD-less mutant (D2-Y160F) of Thermosynechoccocus elongatus were suspended in a high-concentration Mes buffer, and flash-induced FTIR difference spectra were recorded. The Mes bands were extracted from the spectra by subtracting the corresponding spectra recorded in deuterated-Mes (D13-Mes). The flash-number dependence of the intensity increase in the Mes signal showed a typical period-four oscillation. Simulation of this oscillation showed that the proton release pattern was consistent with 1:0:1:2 for the S0->S1->S2->S3->S0 transitions.
