Highly resolved proton matrix ENDOR of oriented photosystem II membranes in the S2 state

•We measured proton matrix ENDOR of S2 state Mn cluster in the oriented photosystem II.•Highly resolved spectra were observed in the oriented photosystem II membranes.•Angular dependence of ENDOR signals was detected in the oriented membranes.•ENDOR signals were assigned to the water protons close to Mn-cluster.•A model for proton conditions in Mn-cluster was proposed.

Proton matrix ENDOR was performed to investigate the protons close to the manganese cluster in oriented samples of photosystem II (PS II). Eight pairs of ENDOR signals were detected in oriented PS II membranes. At an angle of θ = 0° between the membrane normal vector n and the external field H0, five pairs of ENDOR signals were exchangeable in D2O medium and three pairs were not exchangeable in D2O medium. The hyperfine splitting of 3.60 MHz at θ = 0° increased to 3.80 MHz at θ = 90°. The non-exchangeable signals with 1.73 MHz hyperfine splitting at θ = 0°, which were assigned to a proton in an amino acid residue, were not detected at θ = 90° in oriented PS II or in non-oriented PS II. Highly resolved spectra show that only limited numbers of protons were detected by CW-ENDOR spectra, although many protons were located near the CaMn4O5 cluster. The detected exchangeable protons were proposed to arise from the protons belonging to the water molecules, labeled W1-W4 in the 1.9 Å crystal structure, directly ligated to the CaMn4O5 cluster, and nearby amino-acid residue.

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