A comparison of the photochemical activity of two forms of Photosystem II reaction centre isolated from sugar beet

Abstract

Both time-resolved fluorescence and absorption measurements have been conducted on two different forms of Photosystem II reaction centre isolated from sugar beet. One form, called RC IIa, contained 6 chlorophylls and 2 β-carotenes per 2 pheophytins, while the other, called RC Iib, contained 4 chlorophylls and 1 β-carotene per 2 pheophytins. Single photon-counting fluorescence decay obtained from the two preparations showed similar charge recombination fluorescence lifetimes which could be resolved into two components of 46.1 and 14.2 ns. Analysis of the amplitude of the fluorescence of the fast component of 5.6 ns, which largely originates from non-functional chlorophyll, gave an estimate of the relative activity for RC IIb which was only a 5.5% lower compared to that of RC IIa. This small relative difference in photochemical activity was also confirmed by measuring the extent of primary charge separation activity using flash induced absorption spectroscopy. In this case the amplitude of the long-lived component, attributed to primary radical-pair formation and recombination, was 16% lower in RC IIb as compared with RC IIa. When the secondary electron transfer activity of the two forms of reaction centre were measured using MnCl 2 and silicomolibdate as electron donor and acceptor respectively, RC IIb was 16% less active than RC IIa. From the data we conclude that the removal of 2 chlorophylls and 1 β-carotene molecules from the isolated Photosystem II reaction centre only slightly impair its functional activity with respect to primary charge separation. This conclusion seems to suggest that photochemically active isolated reaction centres of Photosystem II and purple bacteria can have the same minimum pigment stoichiometry of 4 chlorophylls and 1 carotenoid per 2 pheophytins.