Electron spin-lattice relaxation studies of different forms of the S(2) state multiline EPR signal of the Photosystem II oxygen-evolving complex.

The pulsed EPR inversion recovery sequence has been utilized to monitor the temperature dependence of the electron spin-lattice relaxation rate of the Mn cluster of the Photosystem II oxygen evolving complex poised in a variety of S (2) state forms giving rise to g = 2 multiline EPR signals. A previous study (Lorigan and Britt (1994) Biochemistry 33: 12072-12076) showed that for PS II membranes treated with 5% ethanol, the S (2) state Mn cluster relaxes via the Orbach spin-lattice relaxation mechanism, where the relaxation is enhanced via phonon scattering off an excited state spin manifold, in this case at an energy of Delta = 36.5 cm(-1) above the S = 1/2 ground state giving rise to the multiline EPR signal. Parallel experiments are reported for PS II membranes with 5% methanol, treated with ammonia, and following short and long term dark adaptation. In each case, the temperature dependence of the electron spin-lattice relaxation rate is consistent with Orbach relaxation, and the range of excited state energies is relatively narrow (33.8 cm(-1) </= Delta </= 39.7 cm(-1)). In addition, short term dark adapted (6 min, 'active state') PS II membranes show biphasic recovery traces which indicate that a minority fraction of the oxygen evolving complexes are trapped in a form with greatly slowed spin-lattice relaxation.