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.

Ultrafast Excitation Energy Dynamics in a Diatom Photosystem I-Antenna Complex: A Femtosecond Fluorescence Upconversion Study.

Fucoxanthin chlorophyll (Chl) a/ c-binding proteins (FCPs) are unique light-harvesting antennas in diatoms. Recent time-resolved fluorescence analysis of photosystem I with FCP associated (PSI-FCPI) has mainly shown excitation energy transfer among Chls a from FCPI to PSI in tens of picoseconds. However, it remains unclear how each pigment, especially carotenoids and Chl c, in the FCPI is functionally related to the energy transfer in a femtosecond time range. Here, we reveal ultrafast excitation energy transfer mechanism in the PSI-FCPI preparations isolated from a diatom, Chaetoceros gracilis, by means of femtosecond time-resolved fluorescence spectroscopy with an upconversion system. Compared with the fluorescence lifetime components of PSI core-like complexes, the energy transfer of Chl c → Chl a in the FCPI was observed within hundreds of femtoseconds, and the energy in the FCPI was transferred to PSI in ∼2 ps. The comparative fluorescence analyses provide physical insights into the energy transfer machinery within FCPI and from FCPI to PSI.