Photosystem II of green plants: on the possible role of retarded protonic relaxation in water oxidation

Photosystem II (PSII) of green plants and cyanobacteria uses energy of ligh t to oxidize water and to produce oxygen. The available estimates of the ox idizing potential of P-680(+), the primary donor of PSII, yield value of ab out 1.15 V. Two main factors are suggested to add up and engender this high oxidizing potential, namely: (1) the electrostatic influence dominated by Arg-181 of the D2 subunit which elevates the oxidizing potential of P680(+) up to 1 V, some 0.1 V above the E-m value of a hydrogen-bonded chlorophyll a; and (2) the dynamic component of 0.10-0.15 V due to the experimentally demonstrated retarded protonic relaxation at the P-680 site. (C) 1999 Elsev ier Science B.V. All rights reserved.