Photochemistry and structural aspects of the photosystem 2 reaction centre

Abstract

In this Thesis, a number of spectroscopic measurements are presented on Photosystem II (PsII) preparations from spinach, which provide some new information on the PSII reaction centre structure and function.The experimental part of this thesis is preceded by a general introduction (Chapter 1) and a brief overview of methods and techniques used (Chapter 2).In Chapter 3, a low temperature Electron Paramagnetic Resonance (EPR) study on PSII-enriched membranes is presented. The redox dependencies of the light-inducible signals from the spin-polarised reaction centre triplet state and signals from the semiquinone-iron (Q A-Fe 2+) acceptor complex were studied. It was found that the reaction centre triplet signal was only seen after the Q A-Fe 2+signal had disappeared on double reduction of Q A .In Chapter 4, a time-resolved fluorescence study at ambient temperature is presented on samples in which Q A was oxidised, singly reduced or doubly reduced. Two types of preparations were used: PSII-enriched membranes (with large antenna size) and PSII core complexes (partly stripped of antenna). The results indicated that the effective rate of charge separation was larger when Q A was oxidised or doubly reduced than when it was singly reduced. In addition, it was found that the lifetime of the primary radical pair state increases when going from the singly reduced state of Q A to the doubly reduced state. The differences between the two types of preparation and the reversibility of the double reduction of Q A were also investigated.In Chapter 5, a more detailed EPR study of the reversibility of the double reduction of Q A in PSII-enriched membranes is presented. Only a small degree of reversibility was found. In addition, the non-heme iron was found to be irreversibly modified (possibly lost) upon double reduction of Q A , which may explain the lack of reversibility.In Chapter 6, the orientation dependence of the reaction centre triplet EPR signal was determined in oriented samples. Both PSII-enriched membranes and D 1 D 2 reaction centre preparations were used. The reaction centre triplet state was found to be localised on a reaction centre chlorophyll of which the tetrapyrrolic plane is oriented at 30° to the membrane plane.In Chapter 7, time-resolved EPR experiments on oriented samples (PSII-enriched membranes) are described. Thus, the risetime of the reaction centre triplet state was estimated to be of the order of 300 - 400 ns.In Chapter 8, low temperature flash-induced absorbance difference measurements are presented on PSII enriched membranes under several redox conditions, including those having Q A singly or doubly reduced. These experiments provided an unexpected explanation for the absence of a detectable EPR triplet signal when Q A is singly reduced (see above): the lifetime is too short to give a measurable signal under continuous illumination. Furthermore, the lifetime of the primary radical pair state was found to increase when going from the singly reduced state of Q A to the doubly reduced state. Finally, low temperature absorbance difference spectra are presented and analysed. These were found to be significantly different from difference spectra in the literature obtained using D 1 D 2 preparations. It was concluded that presently available D 1 D 2 preparations are considerably modified on the donor side compared to PSII-enriched membrane preparations