Electron spin-lattice relaxation studies of different forms of the S2 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 varie ty of S-2 state forms giving rise to g = 2 multiline EPR signals. A previou s 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 rela xes via the Orbach spin-lattice relaxation mechanism, where the relaxation is enhanced via phonon scattering off an excited state spin manifold, in th is 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 s hort and long term dark adaptation. In each case, the temperature dependenc e of the electron spin-lattice relaxation rate is consistent with Orbach re laxation, and the range of excited state energies is relatively narrow (33. 8 cm(-1) less than or equal to Delta less than or equal to 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 th e oxygen evolving complexes are trapped in a form with greatly slowed spin- lattice relaxation.