Ultrafast Carotenoid-to-Chlorophyll Singlet Energy Transfer in the Cytochrome b6f Complex from Bryopsis corticulans

Ultrafast carotenoid-to-chlorophyll (Car-to-Chl) singlet excitation energy transfer in the cytochrome b6f (Cyt b6f) complex from Bryopsis corticulans is investigated by the use of femtosecond time-resolved absorption spectroscopy. For all-trans-α-carotene free in n-hexane, the lifetimes of the two low-lying singlet excited states, S1(2Ag−) and S2(1Bu+), are determined to be 14.3 ± 0.4 ps and 230 ± 10 fs, respectively. For the Cyt b6f complex, to which 9-cis-α-carotene is bound, the lifetime of the S1(2Ag−) state remains unchanged, whereas that of the S2(1Bu+) state is significantly reduced. In addition, a decay-to-rise correlation between the excited-state dynamics of α-carotene and Chl a is clearly observed. This spectroscopic evidence proves that the S2(1Bu+) state is able to transfer electronic excitations to the Qx state of Chl a, whereas the S1(2Ag−) state remains inactive. The time constant and the partial efficiency of the energy transfer are determined to be 240 ± 40 fs and (49 ± 4)%, respectively, which supports the overall efficiency of 24% determined with steady-state fluorescence spectroscopy. A scheme of the α-carotene-to-Chl a singlet energy transfer is proposed based on the excited-state dynamics of the pigments.