A CARBOXYLATE LIGAND INTERACTING WITH WATER IN THE OXYGEN-EVOLVING CENTER OF PHOTOSYSTEM-II AS REVEALED BY FOURIER-TRANSFORM INFRARED-SPECTROSCOPY

The effect of H/D exchange on a Fourier transform infrared (FTIR) difference spectrum between the S-1 and S-2 states of the oxygen-evolving center (OEC) in Photosystem II (PS II) has been investigated. Upon deuteration, a large upshift of a differential band by about 18 cm(-1) was observed in the 1600-1500 cm(-1) region, where asymmetric COO- stretching and amide II modes mainly have intensities. This upshift was basically unaffected by N-15 substitution of the PS II membranes. FTIR measurements of three model carboxylate compounds showed that asymmetric COQ(-) stretching bands upshift upon deuteration when the carboxylate groups form hydrogen bonds with water; for free acetate and Mn-EDTA in which the carboxylate groups interact with bulk water, the upshift was 7 cm(-1), whereas (CH3COO)(2)Ni . 4H(2)O whose carboxylate ligands form hydrogen bonds with water molecules coordinated to the metal ion, exhibits a much larger upshift of 20 cm(-1). The above observations indicate that the most plausible assignment of the upshifting band in the S-2/S-1 spectrum is the asymmetric COO- mode of a carboxylate group interacting with water in OEC. The large upshift of 18 cm(-1) found in PS II may suggest that this carboxylate group, which is possibly a ligand of a redox-active Mn ion, forms a hydrogen bond with a water molecule bound to the Mn-cluster, although the possibility that it forms a strong hydrogen bond with an exchangeable hydrogen atom of the neighboring amino acid residue cannot be excluded. In the amide I region of the S-2/S-1 spectrum, the bands around 1659 cm(-1) exhibited large downshifts to around 1645 cm(-1) upon deuteration. This frequency shift upon deuteration is typical of a random coil structure, and hence it is suggested that hydrophilic domains with random coil conformations are present in the close vicinity of the Mn-cluster.