Photoprotective role of the xanthophyll cycle studied by means of modeling of xanthophyll-LHCII interactions

The problem of photoprotection associated with the xanthophyll cycle is addressed by examination of effects of exogenous violaxanthin and zeaxanthin on isolated antenna complex LHCII. Analysis of electronic absorption spectra suggests multiple sites of binding of violaxanthin and zeaxanthin to the protein environment. Xanthophyll binding results in enhancement of excitonic bands at ca. 530 and 740 nm, as concluded on the basis of Resonance Light Scattering spectra. The energy states attributed to these excitonic bands are concluded to quench singlet excitations. Fluorescence Lifetime Imaging Microscopy shows that both xanthophylls, but in particular zeaxanthin, promote formation of supramolecular structures characterized by extremely low fluorescence yield. Photoprotective meaning of the xanthophyll cycle is concluded to be primarily based upon role of zeaxanthin in promoting formation of supramolecular structures of LHCII characterized by high rate of energy dissipation.