Excitation relaxation dynamics of carotenoids constituting the diadinoxanthin cycle

Carotenoids (Cars) exhibit two functions in photosynthesis, light-harvesting and photoprotective functions, which are performed through the excited states of Cars. Therefore, increasing our knowledge on excitation relaxation dynamics of Cars is important for understanding of the functions of Cars. In light-harvesting complexes, there exist Cars functioning by converting the π-conjugation number in response to light conditions. It is well known that some microalgae have a mechanism controlling the conjugation number of Cars, called as the diadinoxanthin cycle; diadinoxanthin (10 conjugations) is accumulated under low light, whereas diatoxanthin (11 conjugations) appears under high light. However, the excitation relaxation dynamics of these two Cars have not been clarified. In the present study, we investigated excitation relaxation dynamics of diadinoxanthin and diatoxanthin in relation to their functions, by the ultrafast fluorescence spectroscopy. After an excitation to the S₂ state, the intramolecular vibrational redistribution occurs, followed by the internal conversion to the S₁ state. The S₂ lifetimes were analyzed to be 175 fs, 155 fs, and 140 fs in diethyl ether, ethanol, and acetone, respectively, for diadinoxanthin, and 155 fs, 135 fs, and 125 fs in diethyl ether, ethanol, and acetone, respectively for diatoxanthin. By converting diadinoxanthin to diatoxanthin, the absorption spectra shift to longer wavelengths by 5-7 nm, and lifetimes of S₂ and S₁ states decrease by 11-13% and 52%, respectively. Differences in levels and lifetimes of excited states between diadinoxanthin and diatoxanthin are small; therefore, it is suggested that changes in the energy level of chlorophyll a are necessary to efficiently control the functions of the diadinoxanthin cycle.