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 (2(1)A(g)) of diadin oxanthin and diatoxanthin, carotenoids involved in the xanthophyll cyc le in some genera of algae, have been measured by femtosecond time-res olved optical spectroscopy to be 22.8 +/- 0.1 ps and 13.3 +/- 0.1 ps, respectively. Using the energy gap law for radiationless transitions s et forth by Englman and Jortner (Mel. Phys. 18 (1970) 145-164), these lifetimes correspond to S-1 excited state energies of 15210 cm(-1) for diadinoxanthin and 14620 cm(-1) for diatoxanthin. The lowest excited singlet state energy of Chi a has an energy of 14700 cm(-1). The fact that the S-1 state energy of diadinoxanthin lies above that of Chi a, whereas the S-1 state energy of diatoxanthin lies below that of Chi a, suggests that the xanthophyll cycle involving the enzymatic interconv ersion of diadinoxanthin and diatoxanthin may play a role in regulatin g energy flow between these molecules and Chi a in many species of alg ae, essentially fulfilling a role identical to that proposed for viola xanthin and zeaxanthin in higher plants and green algae (Frank et al. (1994) Photosyn. Res. 41, 389-395).