Singlet-Excitation Energy Transfer Among Carotenoids in the HL-LH2 Complex from Rhodopseudomonas Palustris

The structure of peripheral light harvesting complex (LH2) from photosynthetic bacterium Rhodopseudomonas(Rps.) palustris cultured at high-light(HL) is very complicated, which includes five kinds of carotenoids(Cars) molecules in one unit. Femtosecond time-resolved absorption spectroscopy has been used to study the process of excited-state energy transfer(EET) among Cars in LH2 excited by femtosecond laser pulse at different wavelength. The characteristic spectra of singlet (S_n) and triplet (T₁) excitation Cars were observed in visible spectral region. The results suggest that the singlet EET processes may occur from Cars with shorter conjugation length(n) to that with longer conjugation length besides excitation energy internal conversion. These complicated excitation energy equilibrium in the HL-LH2 from Rps. palustris fulfills within 6 ps. It might imply that the coexistence of many Cars is helpful for Cars serving as photoprotection.