The relationship between excitation energy transfer, trapping and antenna size in Photosystem II

Abstract

A systematic study of a range of particles, isolated from both higher plants and the cyanobacterium Synechocystis 6803, has investigated the effect of antenna size on energy transfer and trapping in Photosystem II. Time resolved fluorescence experiments have been employed to obtain the absolute singlet state concentrations as a function of time from each sample, rather than focusing on a multiexponential lifetime analysis of the data. Models that may be used to describe the multiexponential dynamics of energy and electron transfer in isolated reaction centre samples were investigated, to establish whether they could be extended to predict the experimentally observed trapping dynamics in open and closed core particles without the need to vary the reaction centre (`trapping engine¿) parameters. It is demonstrated that macroscopic and microscopic reaction centre `trapping engine¿ models may be extended to describe the experimentally observed trapping dynamics in a range of particles with QA in a fully closed state. This suggests that the presence of the shallow trap and slow charge separation kinetics, observed in isolated D1/D2/cyt b559 reaction centres, are retained in larger particles. Fluorescence decays obtained from cores particles with QA in a fully open state were more difficult to reconcile using macroscopic models without changing the free energy between the excited singlet and primary radical pair states. A microscopic description of the trapping engine may however be used to provide an alternative representation of the dynamics in open core particles that does not require the original parameters employed in the reaction centre trapping engine model to be altered.