Mechanism of nonphotochemical quenching in green plants: Energies of the lowest excited singlet states of violaxanthin and zeaxanthin

The xanthophyll cycle is an enzymatic, reversible process through which the carotenoids violaxanthin, antheraxanthin, and zeaxanthin are interconverted in response to the need to balance light absorption with the capacity to use the energy to drive the reactions of photosynthesis. The cycle is thought to be one of the main avenues for safely dissipating excitation energy absorbed by plants in excess of that needed for photosynthesis. One of the key factors needed to elucidate the molecular mechanism by which the potentially damaging excess energy is dissipated is the energy of the lowest excited singlet (S-1) state of the xanthophyll pigments. Absorption from the ground state (S-0) to S-1 is forbidden by symmetry, making a determination of the S-1 state energies of these molecules by absorption spectroscopy very difficult. Fluorescence spectroscopy is potentially the most direct method for obtaining the S-1 state energies. However, because of problems with sample purity, low emission quantum yields, and detection sensitivity, fluorescence spectra from these molecules, until now, have never been reported. In this work these technical obstacles have been overcome, and S-1 --> S-0 fluorescence spectra of violaxanthin and zeaxanthin are presented. The energies of the S-1 states deduced from the fluorescence spectra are 14 880 +/- 90 cm(-1) for violaxanthin and 14 550 +/- 90 cm(-1) for zeaxanthin. The results provide important insights into the mechanism of nonphotochemical dissipation of excess energy in plants.