The B800–850 complex of the purple bacterium Chromatium tepidum: low-temperature absorption and Stark spectra

Abstract Low-temperature absorption and Stark-effect spectra of the B800–850 complex of Chromatium (C.) tepidum are presented. The low-temperature absorption shows two bands with about equal amplitude, centred at about 795 nm and at about 808 nm. The Stark spectrum of the complex is similar to the Stark spectrum from LH2 complexes from other bacteria, with additional structure around 800 nm. The BChls absorbing at 850 nm possess a difference dipole moment Δ μ → of about 4.3–5.3 D f −1 . The two absorbing species in the 800 nm band give rise to a completely different Stark effect, the 808 nm band having Δ μ → =1.3–1.6 D f −1 similar to that of the B800 band of Rhodopseudomonas acidophila, whereas the 795 band has a much larger Δ μ → (3.6–4.5 D f −1 ). It is suggested that the B800 pigments in LH2 of C . tepidum are present in two forms, which differ in the orientation of their optical Q y -transition moments.