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

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 theta = 0 degrees between the membrane normal vector n and the external field H-0, 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 theta = 0 degrees increased to 3.80 MHz at theta = 90 degrees. The non-exchangeable signals with 1.73 MHz hyperfine splitting at theta = 0 degrees, which were assigned to a proton in an amino acid residue, were not detected at theta = 90 degrees 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 angstrom crystal structure, directly ligated to the CaMn4O5 cluster, and nearby amino-acid residue. (C) 2013 Elsevier B.V. All rights reserved.