Perturbations of exciton and electron transfer within Photosystem II

Abstract

Exciton and electron transfer in Photosystem II (PS II), the membrane protein complex in oxygenic photosynthetic organisms, was studied using flash-induced 830 nm absorption changes which indicate the formation and decay of excited antenna chlorophyll, pheophytin anion (I-) and P680 cation (P680+•). Experimental results were analysed within the framework of existing knowledge about PS II. 1. The P680+• reduction kinetics of thylakoid membrane fragments from the marine prokaryote Prochloron Didemni, a member of the recently discovered group of prochlorophytes, were determined by monitoring flash induced absorption changes at 830 nm and analysing the time course of their decay. The multiphasic relaxation kinetics and their modification by NH2OH were found to be similar to those observed in cyanobacteria and plants. These findings provide an independent line of evidence for the idea of a high conservation of the basic structural and functional pattern of water oxygenic photosynthesis. 2. A theoretical model of the energy and electron transfer kinetics within Photosystem II (PS II) was developed. The model was used to investigate the origin of a decay component with a 1/e life of < 10 ns and the connection between the optical response and the rate of O2 evolution of PS II from Spinacea Oleracea. The extent of bimolecular exciton annihilation and triplet formation via intersystem crossing in the light harvesting complex of PS II excited by saturating flashes of light from a laser was also investigated. 3. The P680+• reduction kinetics of PS II from Spinaceia Oleracea in which the 17 and 23 kDa extrinsic polypeptides are intact, were determined in the presence of Ca2+ or ethylene glycol bis ( [small beta, Greek] -aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA), which were added to vary the Ca2+ concentration from 5 [small mu, Greek] M to 30 mM. The decrease in the extent of normal P680+• reduction decay with lifetimes of 40–370 ns and a corresponding increase in the extent of kinetics with lifetimes of 20–220 [small mu, Greek] s was interpreted as being due to electron transfer from YZ to P680+• being replaced by slow forward conduction and by processes including P680+•/QA- recombination. The question of whether changes in P680+• reduction kinetics were caused by loss of Ca2+ from PS II or by direct interaction of EGTA with PS II was addressed by lowering the free-Ca2+ concentration of suspensions of PS II core complexes by serial dilution in the absence of EGTA. Despite a significant decrease in the rate of O2 evolution after this treatment, only small changes in the P680+• reduction kinetics were observed. Loss of Ca2+ did not affect P680+• reduction associated with electron transfer from YZ. Since much larger changes in the P680+• reduction kinetics of intact PS II occurred at comparable free-Ca2+ concentrations in the presence of EGTA, EGTA influenced the P680+• reduction kinetics by directly interacting with PS II rather than by lowering the free Ca2+ concentration of the surrounding media. 4. The effect of UV-B irradiation on the electron transfer within PS II enriched thylakoid membrane fragments from Spinacea Oleracea was determined by measuring changes to the P680+• reduction kinetics and O2 evolution after irradiation with narrow band UV-B radiation. A significant change in the electron pathway of PSII occurred after irradiation in a narrow band of between 285 and 291 nm, indicating preferred absorption of these wavelengths.