[Selective distribution and physiological function of heterogeneous carotenoids in purple photosynthetic bacterium Rhodopseudomonas palustris].

Purple photosynthetic bacterium Rhodopseudomonas (Rps.) palustris is unique in the heterogeneous carotenoid compositions in peripheral light-harvesting complex (LH2) and core complex (LH1-RC). It is of great significance to investigate selective distribution and the physiological function of various carotenoids occurring in the pigment-protein complexes mentioned above. In the present paper, high performance liquid chromatogram (HPLC) was applied to analyze the carotenoid composition in LH2 and LH1-RC complexes isolated from Rps. palustris growing in medium light intensity. Comparison of the HPLC elution profiles showed that carotenoids having 11 or 12 conjugated double bonds, including lycopene (n = 11), anhydrorhodivibrin (n = 12) and rhodopin (n = 12), were the predominant components in LH2, whereas spirilloxanthin with n = 13 was preferentially bound to core complex. The average yields of carotenoid-to-BChl energy transfer in LH2 and LH1-RC were determined by comparing their steady-state absorption spectra and NIR fluorescence excitation spectroscopy, which were normalized at the BChl Qx transition. It was found that energy transfer yield decreased from 30.9% (upsilon = 0), 25.4% (upsilon = 1) and 30.4% (upsilon = 2) in LH2 complex to 10.7% (upsilon = 0), 7.8% (upsilon = 1) and 11.4% (upsilon = 2) in LH1-RC complex, respectively. Additionally, triplet excited state carotenoids were investigated by means of sub-microsecond time-resolved absorption spectroscopy, where T(n) <-- T1 transient absorption maximum was observed at 571 nm for LH1-RC and 560 nm for LH2 complex at room temperature following the photo-excitation of carotenoid at 532 nm. The dynamic curves of these two spectral components, which were assignable to carotenoids with n = 13 and n = 12, could be well fitted to a single exponential decay function with the apparent time constants of 1.6 and 2.2 micros, respectively. The aforementioned data suggested that heterogeneous carotenoids were selectively distributed in LH2 and core complex, where short-conjugated carotenoids in LH2 complex serve as efficient accessory light-harvesting pigments, whereas long-conjugated carotenoids preferentially bound to LH1-RC mainly perform photo-protection function.