The manganese center of oxygen-evolving and calcium-depleted photosystem II: a pulsed EPR spectroscopy study

The environment of the multi-manganese center in the O2-evolving complex (OEC) of plant photosystem II (PS II) under conditions of Ca2+ depletion has been probed using pulsed electron paramagnetic resonance (EPR) spectroscopy, and the following results are reported: (1) In Ca(2+)-depleted PS II membranes treated with the chelator [ethylenebis(oxyethylenenitrilo)]tetraacetic acid (EGTA), the modified Mn EPR signal arising from the OEC in the S2 state and the split EPR signal from the S3 state could be detected in the absorption mode by recording the amplitude of a two-pulse echo as a function of the external magnetic field. The formation of the S3 signal (g approximately 2.004; delta Hpp = 164 G) is not accompanied by the disappearance of the Mn EPR signal, although the signal becomes difficult to detect in CW EPR. This result supports the previous interpretation of the split S3 EPR signal as arising from the interaction of an organic radical with the Mn cluster [Boussac, A., Zimmermann, J. L., Rutherford, A. W., & Lavergne, J. (1990) Nature 347, 303-306]. (2) The two-pulse electron spin echo envelope modulation (ESEEM) spectra of the S2 state formed in Ca(2+)-depleted PS II membranes obtained from 14N- and 15N-labeled material are different. This indicates that nitrogen nuclei from nitrogen-containing protein residues are coupled to the Mn center in the S2 state of the inhibited enzyme. In addition, comparison with the two-pulse ESEEM data obtained for the S2 state in the untreated enzyme suggests that the coupling may be altered by the Ca2+ depletion and/or EGTA treatment. (3) The treatment of Ca(2+)-depleted PS II membranes with sodium pyrophosphate also induced a stable S2 state characterized by a modified multiline EPR signal that is similar to that obtained in EGTA-treated PS II membranes. Comparison of the ESEEM data obtained for the pyrophosphate and 14N and 15N samples treated with EGTA suggests that the modification induced by the EGTA treatment is accompanied by the binding of (an) EGTA molecule(s) to or near the Mn center. (4) ESEEM data obtained for the S3 state formed in the pyrophosphate or EGTA-treated enzyme are quite similar to those obtained for the corresponding S2 state. The data are also compared with ESEEM data obtained on oxidized 4(5)-methylimidazole obtained by UV irradiation. These results are discussed with respect to the current assignment of the S3 radical as arising from oxidation of a histidine residue.