Abstract
Photosynthetic water oxidation is catalyzed by the tetranuclear Mn-cluster of photosystem (PS) II. Two water molecules are oxidized to an oxygen molecule through five intermediates states (S0-S4). In the present study, flash-induced low-frequency (1000-370 cm-1) FTIR difference spectra for the S-state cycle and effects of H218O-, D216O-, D218O-substitution on the spectra were examined using PS II core particles from Thermosynechococcus elongatus. Low-frequency difference spectra for S1-to-S2, S2-to-S3, S3-to-S0, and S0-to-S1 transitions showed characteristic vibrational features depending on each water-substitution. Close inspection of the isotopic bands revealed that bands only sensitive to 16O/18O exchange or H/D exchange may be ascribed to mainly Mn cluster core vibrations or modes from amino acid side chains and polypeptides. Furthermore, bands sensitive to both 16O/18O and H/D exchanges may be attributed to the interaction between Mn and substrate water and/or water-derived chemical species involved in the water oxidation chemistry.
