The lifetimes and energies of the first excited singlet states of diadinoxanthin and diatoxanthin: The role of these molecules in excess energy dissipation in algae

The lifetimes of the first excited singlet states (2¹A(g)) of diadinoxanthin and diatoxanthin, carotenoids involved in the xanthophyll cycle in some genera of algae, have been measured by femtosecond time-resolved optical spectroscopy to be 22.8 ± 0.1 ps and 13.3 ± 0.1 ps, respectively. Using the energy gap law for radiationless transitions set forth by Englman and Jortner (Mol. Phys. 18 (1970) 145-164), these lifetimes correspond to S₁ excited state energies of 15,210 cm^-1 for diadinoxanthin and 14,620 cm^-1 for diatoxanthin. The lowest excited singlet state energy of Chl a has an energy of 14,700 cm^-1. The fact that the S₁ state energy of diadinoxanthin lies above that of Chl a, whereas the S₁ state energy of diatoxanthin lies below that of Chl a, suggests that the xanthophyll cycle involving the enzymatic interconversion of diadinoxanthin and diatoxanthin may play a role in regulating energy flow between these molecules and Chl a in many species of algae, essentially fulfilling a role identical to that proposed for violaxanthin and zeaxanthin in higher plants and green algae (Frank et al. (1994) Photosyn. Res. 41, 389-395).