Abstract
In the oxygen-evolving complex of PSII, two water molecules are cleaved into four protons and one molecular oxygen through a light-driven cycle of S states (S0-S4). To study the proton release steps in this cycle, we have examined the pH dependence of the S-state transitions using FTIR spectroscopy. FTIR difference spectra upon 1st- to 4th-flash illumination were measured using PSII core complexes from Thermosynechococcus elongatus at pH4-10. The efficiencies of S-state transitions at each pH were estimated by fitting the spectra with the standard spectra measured at pH6. The results showed that the efficiencies of S2->S3, S3->S0 and S0->S1 transitions decreased by lowering pH, but were less sensitive at higher pHs. This is consistent with the view that protons are released in the S2->S3, S3->S0 and S0->S1 transitions.
