SPECTRAL RESOLUTION OF MORE THAN ONE CHLOROPHYLL ELECTRON-DONOR IN THE ISOLATED PHOTOSYSTEM-II REACTION CENTER COMPLEX

Prolonged illumination of the isolated Photosystem II reaction centre under aerobic conditions causes a selective photodestruction of chlorophyll which absorbs maximally at 680 nm. Concomitant with this effect is a loss of photochemical activity. When oxygen is absent, the reaction centre is no longer damaged by illumination. Protection of the 680 nm absorbing chlorophyll against photodamage and maintenance of photochemical activity can also be achieved if silicomolybdate (SiMo) is present as an electron acceptor, although in this case there is an irreversible bleaching at 670 nm, whether oxygen is present or not. We therefore suggest that there are two chlorophyll species in the reaction centre absorbing at 670 nm and 680 nm, respectively. The latter we attribute to the primary donor P680 and the former to an accessory chlorophyll. It seems highly likely that in the absence of SiMo but under aerobic conditions, the photodestruction of P680 involves singlet oxygen generated from the P680 triplet. When SiMo is present the yield of the triplet is significantly reduced (Nugent, J.H.A., Telfer, A., Demetriou, C. and Barber, J. (1989) FEBS Lett. 255, 53-58) due to electron transfer to the acceptor and thus this mode of photodegradation is reduced. However, the accumulation of P680+ when SiMo is present facilitates the oxidation of the accessory 670 nm chlorophyll which seems to result in its photodestruction by a mechanism not involving oxygen. © 1990.