Lutein can act as a switchable charge-transfer quencher in the CP26 light-harvesting complex.

Energy-dependent quenching of excitons in Photosystem II of plants, or qE, has been positively correlated with the transient production of carotenoid radical cation species. Zeaxanthin was shown to be the donor species in the CP29 antenna complex. We report transient absorbance analyses of CP24 and CP26 complexes that bind lutein and zeaxanthin in the L1 and L2 domains, respectively. For CP24 complexes, the transient absorbance difference profiles give a reconstructed transient absorbance spectrum with a single peak centered at ~980 nm, consistent with zeaxanthin radical cation formation. In contrast CP26 gives constants for the decay components probed at 940 nm and 980 nm of 144 ps and 194 ps, a transient absorbance spectrum that has a main peak at 980 nm, and a substantial shoulder at 940 nm. This suggests the presence of two charge-transfer quenching sites in CP26 involving zeaxanthin radical cation and lutein radical cation species. We also show that lutein radical cation formation CP26 is dependent on binding of zeaxanthin to the L2 domain, implying that zeaxanthin acts as an allosteric effector of charge-transfer quenching involving lutein in the L1 domain.