EPR AND ENDOR STUDIES OF THE WATER OXIDIZING COMPLEX OF PHOTOSYSTEM-II

A comparative study of X-band EPR and ENDOR of the S-2 State of photos ystem II membrane fragments and core complexes in the frozen state is presented. The S-2 state was generated either by continuous illuminati on at T = 200 K or by a single turn-over light flash at T = 273 K yiel ding entirely the same S-2 state EPR signals at 10 K. In membrane frag ments and core complex preparations both the multiline and the g = 4.1 signals were detected with comparable relative intensity. The absence of the 17 and 23 kDa proteins in the core complex preparation has no effect on the appearance of the EPR signals. H-1-ENDOR experiments per formed at two different field positions of the S-2 state multiline sig nal of core complexes permitted the resolution of four hyperfine (hf) splittings. The hf coupling constants obtained are 4.0, 2.3, 1.1 and 0 .6 MHz, in good agreement with results that were previously reported ( Tang et al. (1993) J Am Chem Soc 115: 2382-2389). The intensities of a ll four line pairs belonging to these hf couplings are diminished in D 2O. A novel model is presented and on the basis of the two largest hfc 's distances between the manganese ions and the exchangeable protons a re deduced. The interpretation of the ENDOR data indicates that these hf couplings might arise from water which is directly ligated to the m anganese of the water oxidizing complex in redox state S-2.