QUENCHING OF CHLOROPHYLL-A SINGLETS AND TRIPLETS BY CAROTENOIDS IN LIGHT-HARVESTING COMPLEX OF PHOTOSYSTEM-II - COMPARISON OF AGGREGATES WITH TRIMERS

Laser-induced changes in the absorption spectra of isolated light-harv esting chlorophyll a/b complex (LHC II) associated with photosystem II of higher plants have been recorded under anaerobic conditions and at ambient temperature by using multichannel detection with sub-microsec ond time resolution. Difference spectra (Delta A) of LHC II aggregates have been found to differ from the corresponding spectra of trimers o n two counts: (i) in the aggregates, the carotenoid (Car) triplet-trip let absorption band (Delta A > 0) is red-shifted and broader; and (ii) the features attributable to the perturbation of the Q(y) band of a c hlorophyll a (Chla) by a nearby Car triplet are more pronounced, than in trimers. Aggregation, which is known to be accompanied by a reducti on in the fluorescence yield of Chla, is shown to cause a parallel dec line in the triplet formation yield of Chla; on the other hand, the ef ficiency (100%) of Chla-to-Car transfer of triplet energy and the life time (9.3 mu s) of Car triplets are not affected by aggregation. These findings are rationalized by postulating that the antenna Cars transa ct, besides light-harvesting and photoprotection, a third process: ene rgy dissipation within the antenna. The suggestion is advanced that lu teins, which are buried inside the LHC II monomers, as well as the oth er, peripheral, xanthophylls (neoxanthin and violaxanthin) quench the excited singlet state of Chla by catalyzing internal conversion, a dec ay channel that competes with fluorescence and intersystem crossing; s upport for this explanation is presented by recalling reports of simil ar behaviour in bichromophoric model compounds in which one moiety is a Car and the other a porphyrin or a pyropheophorbide. (C) 1997 Elsevi er Science B.V.