Chlorophyll fluorescence quenching by xanthophylls

The interactions of the xanthophylls zeaxanthin, antheraxanthin and violaxanthin with chlorophyll, which are relevant for the chlorophyll fluorescence quenching in the non-photochemical quenching (NPQ) process in green plants, are investigated by means of quantum chemical methods. In particular, we use a hybrid approach consisting of time-dependent density-functional theory (TDDFT) and configuration interactions singles (CIS), since present-day TDDFT alone fails in describing relevant long-range charge-transfer states. Calculation of the energetically lowest excited states of a zea-chl dimer along the intermolecular distance coordinate shows that two mechanisms for chlorophyll fluorescence quenching are possible: quenching via excitation-energy transfer and electron-transfer quenching. The relevance of both mechanisms is discussed by comparison to corresponding anthera-chl and vio-chl dimers and the dependence of the mechanism on the geometrical arrangement of the dimers is illuminated. It is pointed out that the typical absorption band of the zeaxanthin radical cation can be used to experimentally determine whether a zea-chl dimer is present during NPQ.