Abstract
Structural changes of α-carboxylate of the D1 C-terminal alanine-344 (Ala344) during S-state cycling of photosynthetic oxygen evolving complex were selectively measured using light-induced Fourier transform infrared (FTIR) difference spectroscopy with specific 13C-labeling of α-carboxylate of Ala in photosystem II core particles from Synechocystis sp. PCC 6803. Several bands effected by 13C-labeling in S2/S1 FTIR difference spectrum at 1360-1300 cm-1 were assigned as those originated from the C-terminal α-carboxylate. The bands showed characteristic changes during S-state cycling. They appeared prominently upon the S1-to-S2 transition and to a lesser extent upon the S2-to-S3 transition, but reappeared with the opposite sign upon the S3-to-S0 transition. No obvious isotopic band appeared upon the S0-to-S1 transition. These results indicate that the α-carboxylate of C-terminal Ala344 ligates, in a unidentate manner, a Mn ion that is oxidized upon the S1-to-S2 transition and reduced reversely upon the S3-to-S0 transition.
