A photoprotection mechanism involving the D2 branch in photosystem II cores with closed reaction centers

Abstract

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 β-carotene (³Car) 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 μs and is formed with a quantum yield between 10 and 20%. The ³Car signal is assigned to the β-carotene on the D₂ 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 (³Chl) is formed by the radical pair (RP) mechanism on the normally inactive D₂ branch where it can be subsequently quenched by the D₂β-carotene. We suggest that the D₂ branch becomes active when the RCs are closed under high light fluence conditions. Under these conditions the D₂ 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 D₁-D₂-cyt b₅₅₉ 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 ³Chl is formed on the D₁ branch. The D₁ branch ³Chl can not be quenched due to the large distance to the β-carotene. This interpretation is actually in line with the well-known ³RC quenching mechanism in bacterial RCs, where also the carotenoid in the (analogous to the D₂ branch) B-branch of the RC becomes the quencher.