Triplet state involvement in primary photochemistry of photosynthetic photosystem II

Abstract

THE primary photochemical reaction of photosynthesis is widely accepted to be a one-electron transfer from a chlorophyll (or bacteriochlorophyll) species to an acceptor entity¹. Until recently it had been assumed that the excited state precursor to the electron-transfer reaction was a singlet state in each of the photosynthetic photosystems. This view is based on the fact that estimated fluorescence lifetimes² of ∼ 7 ps at the reaction centre are much too short compared with intersystem crossing times of the order of 5 ns for chlorophyll in vitro³. Robinson³ pointed out, however, that the increase in fluorescence yield observed when the photosystem II (PS II) trap is closed is far less than that expected if photochemistry, such as electron transfer, were the sole quenching process. He and others^4,5 have proposed the involvement of a triplet state in the mechanism of the photochemical electron transfer, but this suggestion has received little attention. Blankenship et al.⁶ reported an electron spin resonance (ESR) signal arising from photosystem I (PS I) which initially exhibited microwave emission due to chemically induced dynamic electron polarisation (CIDEP). They maintained that this observation indicated that the excited state precursor to the formation of the radical species must have been in a triplet state to account for the CIDEP phenomenon. We report here the first observation of ESR signals exhibiting the CIDEP phenomenon from free radicals associated with PS II of chloroplasts and whole algae. As discussed below, this provides additional evidence that triplet states may be involved as intermediates in the mechanism of photochemical electron transfer in photosynthesis.