Triplet state dynamics of chlorophylls in photosynthetic reaction centers and model systems

Abstract

In this work the temperature dependence of the lineshape, and more specifically, the electron spin polarization pattern of the Δm = ±1 triplet EPR (Electron Paramagnetic Resonance) spectra from several photosynthetic purple bacteria has been investigated.In Chapter I a general introduction is presented to photosynthesis, focussing on the structure of a the protein complex, known as the reaction center (RC), and the processes in this RC. All investigations presented in this work are related to processes arid magnetic interactions within this RC. At the end of Chapter I the triplet state (P R) of the primary electron donor (P) is introduced, and also the way it is generated, i.e. in sufficiently high magnetic fields via the radical pair mechanism.In Chapter II the theoretical background is presented to support the explanations given in later chapters in order to understand the experimental results. First it is proven that the observed temperature dependence of the triplet lineshape cannot be caused by intramolecular spin-lattice relaxation. Then the radical pair state P Fis generalized to a three electron spin state, coupled to the paramagnetic Fe 2+ion in the RC. Solving the secular equations exactly for a Hamiltonian which contains all relatively strong magnetic interactions, the time-evolution of P Fis obtained, resulting in the break-down of the exclusive S-T o mixing, which is characteristic for the radical pair mechanism. The effect of spin transitions on the Fe 2+is considered phenomenologically. The theory developed in this Chapter is used, and in some respects extended in Chapter V.In Chapter III time-resolving equipment and experimental techniques are described, as far as employed in this work.The design and construction of a high-gain, broad-band tunable pulsed dye laser is reported. Furthermore, a novel time-resolving EPR technique is described, which is based on broad-band phase-sensitive detection. The technique is able to detect EPR transients with frequency components from 0 Hz - 1 MHz and a low Initial SIN ratio, which makes it very suitable for the purpose of detecting the time-evolution of the triplet state in photosynthetic RC's. Furthermore, a means is described to reduce the light-induced transient artefact distorting time- resolved EPR signals, because of the high-power laser flash.In Chapter IV the results are presented from a number of studies on the lineshape of EPR spectra, reflecting steady state populations of P R, as a function of: i) temperature, ii) redox potential, iii) SDS incubation, or iv) modification of the electron acceptor side in the RC. The results were interpreted as follows: The lineshape reflects the population distribution over the spin levels of P R. This distribution is determined by the processes in the precursor state P F. The relaxation processes of the paramagnetic Fe 2+are transferred by the unpaired electron spin of the reduced primary quinone. The latter is magnetically coupled to the familiar radical pair in the RC. Thus the primary quinone. acts as a relaxation carrier within P F.In Chapter V the explanation presented in the preceding Chapter, is considered with respect to its time-evolution. Time-resolved EPR measurements established that the (temperature dependent) lineshape, as observed under steady-state conditions, Is already present In the first microsecond after excitation at 100 K. This Indicated that the processes within P Ffavour recombination of P +with I -(I denotes the intermediary acceptor) into P Rinto a T + or T - spin configuration for magnetic fields parallel to the Y triplet axis of P R. Using the theoretical results of Chapter II, the preferential T ± recombination probability was qualitatively explained.Once P Rhas been generated, the time-evolution. of the EPR triplet signals shows a remarkable temperature dependence. Spin-lattice relaxation seems to be dependent on temperature only for T < 40 K. A model is proposed to explain this unusual temperature dependence. In this model transitions from P Rback into P Fare invoked.The values of the P Rdecay rate constants at 4 K are found to be independent of temperature up to at least 120 K. Furthermore, the relaxation rate as measured as an increased triplet decay is found to be proportional with the square of the temperature.In Chapter VI the use of the lineshape of P Rtriplet EPR spectra was demonstrated in a series of experiments in which the RC's from Rhodopseudomonasviridis were incorporated into a reversed micellat solution (water in oil). One of the results is that under very strict conditions the RC maintains its native structure. Damage to the acceptor side (which turned out to be the most fragile part of the RC) is reflected by pronounced changes in the lineshape of the EPR spectrum, monitored at - 100 K