A COMPARISON OF THE PHOTOCHEMICAL ACTIVITY OF 2 FORMS OF PHOTOSYSTEM-II REACTION-CENTER ISOLATED FROM SUGAR-BEET

Both time-resolved fluorescence and absorption measurements have been conducted on two different forms of Photosystem II reaction centre iso lated from sugar beet. One form, called RC IIa, contained 6 chlorophyl ls and 2 beta-carotenes per 2 pheophytins, while the other, called RC IIb, contained 4 chlorophylls and 1 beta-carotene per 2 pheophytins. S ingle photon-counting fluorescence decay obtained from the two prepara tions showed similar charge recombination fluorescence lifetimes which could be resolved into two components of 46.1 and 14.2 ns. Analysis o f the amplitude of the fluorescence of the fast component of 5.6 ns, w hich largely originates from non-functional chlorophyll, gave an estim ate of the relative activity for RC IIb which was only a 5.5% lower co mpared to that of RC IIa. This small relative difference in photochemi cal activity was also confirmed by measuring the extent of primary cha rge separation activity using flash induced absorption spectroscopy. I n this case the amplitude of the long-lived component, attributed to p rimary radical-pair formation and recombination, was 16% lower in RC I Ib as compared with RC IIa. When the secondary electron transfer activ ity of the two forms of reaction centre were measured using MnCl2 and silicomolibdate as electron donor and acceptor respectively, RC IIb wa s 16% less active than RC IIa. From the data we conclude that the remo val of 2 chlorophylls and 1 beta-carotene molecules from the isolated Photosystem II reaction centre only slightly impair its functional act ivity with respect to primary charge separation. This conclusion seems to suggest that photochemically active isolated reaction centres of P hotosystem II and purple bacteria can have the same minimum pigment st oichiometry of 4 chlorophylls and 1 carotenoid per 2 pheophytins.