Chloride Cofactor in the Photosynthetic Oxygen-Evolving Complex Studied by Fourier Transform Infrared Spectroscopy

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Abstract
Fourier transform infrared (FTIR) spectroscopy, using midfrequency S2/S1 FTIR difference spectra, has been applied to studies of chloride cofactor in the photosynthetic oxygen-evolving complex (OEC) to determine the effects of Cl- depletion and monovalent anion substitution. Cl- depletion resulted in the disappearance of a large part of the amide I and II vibrational modes, and induced characteristic modification in the features of the stretching modes of the carboxylate ligands of the Mn cluster. The normal spectral features were largely restored by replenishment of Cl- except for some changes in amide bands. The overall features of Br--, I--, or NO3--substituted spectra were similar to those of the Cl--reconstituted spectrum, consistent with their ability to support oxygen evolution. In contrast, the spectrum was significantly altered by the replacement of Cl- with F- or CH3COO-, which resulted in marked suppression and distortion of both the carboxylate and amide bands. The activity of oxygen evolution restored by NO3- was as high as that by Cl- when measured under limited light conditions, indicating that the NO3--substituted OEC is fully active in oxygen evolution, although with a slow turnover rate. The double-difference spectrum between the 14NO3--substituted and 15NO3--substituted S2/S1 difference spectrum showed isotopic bands for asymmetric NO stretching mode in the region of 1400−1300 cm-1 due to NO3- bound to the Cl- site. This demonstrated structural coupling between the Cl- site and the Mn cluster. A proposed model for the isotopic bands suggested that Cl- as well as NO3- is not directly associated with the Mn cluster and exists in a more symmetric configuration and weaker binding state in the S2 state than in the S1 state. These results also suggest that Cl- is required for changes in the structure of the specific carboxylate ligand of the Mn cluster as well as the peptide backbone of protein matrixes upon the transition from S1 to S2.