Structural Coupling of an Arginine Side Chain with the Oxygen-Evolving Mn₄Ca Cluster in Photosystem II As Revealed by Isotope-Edited Fourier Transform Infrared Spectroscopy

Photosynthetic oxygen evolution by plants, algae, and cyanobacteria is performed at the Mn₄Ca cluster in photosystem II (PSII) by light-driven water oxidation. It has been proposed that CP43-Arg357, which is located in the vicinity of the Mn₄Ca cluster, plays a key role in the O₂ evolution mechanism; however, direct evidence for its involvement in the reaction has not yet been obtained. In this study, we have for the first time detected the structural coupling of CP43-Arg357 with the Mn₄Ca cluster by means of isotope-edited Fourier transform infrared (FTIR) spectroscopy. Light-induced FTIR difference spectra upon the S₁→S₂ transition (S₂/S₁ difference spectra) of the Mn₄Ca cluster were measured using isolated PSII core complexes from Synechocystis sp. PCC 6803 cells, where the Arg side chains were labeled with either [η_1,2-¹⁵N₂]Arg or [ζ-¹³C]Arg. Bands due to Arg side chain vibrations, which were extracted by taking a double difference between the S₂/S₁ spectra of isotope-labeled and unlabeled samples, were found at 1700−1600 and 1700−1550 cm⁻¹ for [η_1,2-¹⁵N₂]Arg- and [ζ-¹³C]Arg-labeled PSII, respectively. These frequency regions are in good agreement with those of the CN/NH₂ vibrations of a guanidinium group in difference spectra between isotope-labeled and unlabeled Arg in aqueous solutions. The detected Arg bands in the S₂/S₁ difference spectra were attributed to CP43-Arg357, which is the only Arg residue located near the Mn₄Ca cluster. The presence of relatively high frequency bands arising from unlabeled Arg suggested that the guanidinium N_ηH₂ is engaged in strong hydrogen bonding. These results indicate that CP43-Arg357 interacts with the Mn₄Ca cluster probably through direct hydrogen bonding to a first coordination shell ligand of a redox-active Mn ion. This structural coupling of CP43-Arg357 may play a crucial role in the water oxidation reactions.