Spectroscopic properties of LH2 from Thermochromatium tepidum in liposome and detergent micelles

By the use of dynamic light scattering(DLS), transmission electron microscopy(TEM), UV-Vis and resonance Raman spectroscopy, the properties of LH2 from Thermochromatium(Tch.) tepidum in two detergent micelles and in liposome were studied, respectively. The results show that in LH2-liposome the conformation of spirilloxanthin, an incorporated longer chain carotenoid, is obviously different from that in detergent micelles. The charge state of the terminal groups of detergents or lipids is the key factor affecting the B850 absorption. Generally, the calcium ions cause red-shift of the B850 absorption and hyperchromicity, while the proton has the opposite effect. Based on the amino acid sequence analysis, several amino acid residues on the C-terminus of α apo-protein were proposed to constitute the potential binding site for the calcium ion and proton. Such Ca²⁺- and H⁺-regulated changes of B850 absorption might be an adaptive mechanism to the living environment for this species.