PARENT SESSION
Posters P7A Mechanisms of water oxidation. Abstracts (347-381)

Water oxidation by PSII - relaxation of the potential of the tyrosine-z radical prior to the oxygen-formation step analyzed by delayed fluorescence measurements. Markus Grabolle, Holger Dau^*,1, ¹ Free Univ. Berlin, D-14185 Berlin, Germany

ABSTRACT- Microsecond components of the P680+ reduction have been repeatedly observed and recently discussed in terms of a slow relaxation of the potential of the Tyr-Z+. In the present work, using highly active PSII membrane particles, the free energy difference between the excited antenna state and the reached bi-radical state is studied by a quantitative analysis of the recombination fluorescence emitted at 10 s - 10 ms after excitation by saturating ns-Laser flashes (532 nm). A slow, multiphasic relaxation at the [Qa-, Z+] level by about 100 meV is observed prior to the S4=>S0 transition; the investigated relaxation proceeds in the time range from 10-200 s. The pH and temperature dependence of the relaxation rates and of the extent of this energetic relaxation (G_relax) were determined. The G_relax is found to be pH-dependent (about 30 meV per pH unit) and to exhibit a sizable H/D-effect. The temperature dependence of G_relax suggests an entropically driven process. We propose that the relaxation process involves both, proton release into the bulk phase as well as specific inner-protein proton shifts. The putative mechanistic relevance is discussed in conjunction with recent results obtained by time-resolved X-ray absorption spectroscopy on the S3=>S4=>S0 transition (see also contribution of Haumann et al.).

KEY WORDS: chlorophyll fluorescence, oxygen evolution, delayed fluorescence, proton release