Identification of a possible Mn-O-Mn cluster vibrational mode of the S3 state in the oxygen-evolving complex of photosystem II by the low-frequency FTIR spectroscopy

Abstract

We have developed conditions for recording the low-frequency S2/S1 FTIR spectrum of hydrated PSII samples [Chu, H.-A., Sackett, H. and Babcock, G.T. (2000) Biochemistry 39, 14371-14376]. By exchanging PSII samples with buffered 18O water and by exchanging Ca2+ with Sr2+ and 44Ca2+, we are able to assign the S2 state mode at 606 cm-1 and a corresponding S1 mode at about 625 cm-1 to a Mn-O-Mn cluster vibrational mode of the oxygen-evolving complex (OEC) in PSII. Our results indicate that the bridged oxygen atom in this Mn-O-Mn cluster is exchangeable and accessible by water. Our results also indicate that Sr2+ substitution in PSII causes a small structural perturbation that affects the bond strength of the Mn-O-Mn cluster. In addition, we also found that the Mn-O-Mn cluster mode at 606 cm-1 in the low-frequency Synechocystis wild-type spectrum shifts to 612 cm-1 in the D1-D170H mutant spectrum [Chu, H.-A., Debus, R.J. and Babcock, G.T. (2001) Biochemistry 40, 2312-2316]. We conclude that D1-Asp170 either directly ligates Mn or Ca2+ or participates in a hydrogen bond to the Mn4Ca2+ cluster. Currently, we are in the process of examining other site-directed mutants and of adding isotopic labeling to this integrated approach as part of a strategy to determine the structural mechanism of photosynthetic water oxidation.