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 FTIR difference spectra for the S-state cycle and effects of H218O-substitution on the spectra were examined using PS II core particles from Thermosynechococcus elongatus. Mid-frequency (1800 - 1000 cm-1) difference spectra for S1-to-S2, S2-to-S3, S3-to-S0, and S0-to-S1 transitions showed characteristic vibrational modes from backbone polypeptides and amino acid residues associating with the Mn-cluster, but were scarcely affected by H218O-substitution. In low-frequency (650 - 350 cm-1) difference spectra, 650 - 600 cm-1 bands changed in their sign and intensity during the S-state transitions and showed considerable isotope shifts upon H218O-substitution, indicating that these bands reflect the changes of the Mn-cluster core modes and/or Mn-water modes that are involved in the water oxidation chemistry.
