PARENT SESSION
Posters P4Ab Type II reaction centres: Acceptor side. Abstracts (272-288)

A high-field electron paramagnetic resonance study of the primary electron acceptor of Photosystem II generated under cryogenic conditions. Charilaos Goussias^{*,1, 2}, A. William Rutherford¹, Sun Un¹, ¹ Service de Bioenergetique, Gif-sur-Yvette, France² Institute of Materials Science, Athens, Greece

ABSTRACT- This high-field EPR study was performed in order to gain some insight on the electrostatic environment around the primary semiquinone acceptor of Photosystem II. Semiquinones in solvent solution were found to be sensitive to local electrostatic effects. In PSII the EPR spectra of photochemically or chemically reduced Q_A^-. in iron-depleted PSII showed a g-value shift of 1x10-4 in the g_x component when the pH was changed from 6.0 to 8.5. No further change in the semiquinone spectrum occurred in the range pH 8.5 to pH 4.0. We have shown recently that tyrosyl radical formation is accompanied by a change in the electrostaic environment associated with deprotonation and/or protein relaxation and that these processes could be blocked by generation of the radical at liquid helium temperatures (P. Faller, Ch. Goussias, A.W. Rutherford, S. Un, 2003, Proc. Natl. Acad. Sci. U.S.A., 100, 8732-8735). We report here that no such changes occurred with the Q_A^-: an identical spectrum was obtained, whether generated at 1.8K or at room temperature. This suggests that, unlike the TyrD site, the Q_A binding site is "designed" exclusively for electron transfer involving the formation an anionic radical without compensatory proton or protein movements in its vicinity. We will also present data showing that the Q_A^- generated by illumination at 2K undergoes charge recombination with the "strained" form of tyrosine D radical.

KEY WORDS: QA, high-field EPR, charge recombination