Excited-state energy level does not determine the differential effect of violaxanthin and zeaxanthin on chlorophyll fluorescence quenching in the isolated light-harvesting complex of photosystem II

The mechanism of action of xanthophyll cycle carotenoids in controlling the quenching of chlorophyll fluorescence in the major light-harvesting comple x of photosystem II (LHCIIb) has been investigated. Auroxanthin, a diepoxy carotenoid with 7 conjugated carbon double bonds, violaxanthin (9 conjugate d double bonds) and zeaxanthin (11 conjugated double bonds) have been compa red with regard to their effects in vitro on fluorescence quenching and LHC IIb oligomerization. It was found that auroxanthin stimulated fluorescence quenching, similar to the effect of zeaxanthin and in contrast to the inhib ition caused by violaxanthin, Auroxanthin caused an increase in the oligome rization of LHCIlb and an increase in relative emission of long-wavelength fluorescence at 77 K, It is concluded that auroxanthin can mimic the effect of zeaxanthin on LHCII, strongly suggesting that the xanthophyll cycle car otenoids control quenching in vitro by an indirect structural effect and no t by direct quenching of chlorophyll excited states.