Secrets of carotenoid binding

A photoprotection mechanism involving the D(2) branch in photosystem II cores with closed reaction centers

Nanosecond transient absorption spectroscopy has been used to study reaction centre (RC) chlorophyll triplet quenching by carotenoid in intact photosystem II cores from T. elongatus with closed RCs. We found a triplet beta-carotene ((3)Car) signal (absorption difference maximum at 530 nm) that is sensitized by the RC chlorophyll (Chl) triplet with a formation time of ca. 190 ns, has a decay time of 7 micros and is formed with a quantum yield between 10 and 20%. The (3)Car signal is assigned to the beta-carotene on the D(2) branch of the RC. We thus propose a new photoprotection mechanism operative in closed RCs where-as a consequence of the negative charge on the quinone Q(A)-the triplet chlorophyll ((3)Chl) is formed by the radical pair (RP) mechanism on the normally inactive D(2) branch where it can be subsequently quenched by the D(2) beta-carotene. We suggest that the D(2) branch becomes active when the RCs are closed under high light fluence conditions. Under these conditions the D(2) branch plays a photoprotective role. This interpretation allows combining many seemingly inconsistent observations in the literature and reveals the so far missing RC triplet quenching mechanism in photosystem II. The newly proposed mechanism also explains the reason why this RC triplet quenching is not observed in isolated D(1)-D(2)-cyt b(559) RCs. If Q(A) is either not present at all (as in the isolated RC) or is not charged (as in open RCs or with doubly reduced Q(A)) then the RC (3)Chl is formed on the D(1) branch. The D(1) branch (3)Chl can not be quenched due to the large distance to the beta-carotene. This interpretation is actually in line with the well-known (3)RC quenching mechanism in bacterial RCs, where also the carotenoid in the (analogous to the D(2) branch) B-branch of the RC becomes the quencher.