Excitation Dynamics of Two Spectral Forms of the Core Complexes from Photosynthetic Bacterium Thermochromatium tepidum

The intact core antenna-reaction center (LH1-RC) core complex of thermophilic photosynthetic bacterium Thermochromatium (Tch.) tepidum is peculiar in its long-wavelength LH1-Qy absorption (915 nm). We have attempted comparative studies on the excitation dynamics of bacteriochlorophyll (BChl) and carotenoid (Car) between the intact core complex and the EDTA-treated one with the Qy absorption at 889 nm. For both spectral forms, the overall Car-to-BChl excitation energy transfer efficiency is determined to be ∼20%, which is considerably lower than the reported values, e.g., ∼35%, for other photosynthetic purple bacteria containing the same kind of Car (spirilloxanthin). The RC trapping time constants are found to be 50∼60 ps (170∼200 ps) for RC in open (closed) state irrespective to the spectral forms and the wavelengths of Qy excitation. Despite the low-energy LH1-Qy absorption, the RC trapping time are comparable to those reported for other photosynthetic bacteria with normal LH1-Qy absorption at 880 nm. Selective excitation to Car results in distinct differences in the Qy-bleaching dynamics between the two different spectral forms. This, together with the Car band-shift signals in response to Qy excitation, reveals the presence of two major groups of BChls in the LH1 of Tch. tepidum with a spectral heterogeneity of ∼240 cm−1, as well as an alteration in BChl-Car geometry in the 889-nm preparation with respect to the native one.