PARENT SESSION
Posters P2A Type I reaction centres. Abstracts (181-218)

Measurement of electron transfer via both phylloquinones in the Photosystem I reaction centre of Chlamydomonas. Peter Heathcote^*,1, Stefano Santabarbara¹, Ilya Kuprov², Wendy Fairclough¹, Peter Hore², Saul Purton³, Michael Evans³, ¹ School of Biological Sciences, London, UK² Department of Chemistry, Oxford, UK³ Department of Biology, London, UK

ABSTRACT- The electron spin echo envelope modulation (ESEEM) of out-of-phase (OOP) electron spin echo signals from the geminate radical pair formed by charge separation between P700 and phylloquinone has been studied in thylakoids prepared from the wild-type strain of C. reinhardtii and in two site-directed mutants, in which the methionine residue which acts as the axial ligand to the chlorin electron acceptor A₀ has been substituted with a histidine either on the PsaA (PsaA-M684H) or the PsaB (PsaB-M664H) reaction centre subunits. Two different distances between the unpaired electron spin density clouds of the cation of the primary donor P700 and the phyllosemiquinone are determined. In mutants of the PsaA methionine in which the PsaB radical pair is expected to be observed the distance is 24.27A. In mutants of the PsaB methionine in which the PsaA side radical pair should be seen the distance is 25.43A. We conclude that different radical pairs are seen in the mutants indicating the charge separation can occur on each side of the reaction centre. In wild type photosystem I preparations in which the Fe-Sx centre is reduced an average of the two radical pairs distances is observed. We have also constructed site-directed mutants of residues in the binding pocket of the PsaB branch phyllosemiquinone, and results from analysis of these mutants will be presented.

KEY WORDS: bi-directionality, ESEEM, photosystem I, phylloquinones