Low temperature excitation and emission spectroscopy of the photosynthetic bacteria Rhodopseudomonas sphaeroides 'wild-type' strain ATCC 17023.

Prompt and delayed emission of Rhodopseudomonas sphaeroides 'wild-type' strain ATCC 17023 was measured in the spectral range 550-950 nm at 1.7 K. The broad emission spectra could be resolved into separate bands by excitation spectroscopy. Through a comparison of the separated excitation and emission spectra of the present study with absorption and fluorescence spectra of photosynthetic pigments given in the literature, the emitting species could be identified. Beside the well-known pigments bacteriochlorophyll (BChl), bacteriopheophytin (BPh) and carotenoids, additional pigments could be detected. In the visible and near-infrared range, the fluorescence of pigments could be observed which were produced by the biosynthesis of BChl in the cells. Further, fluorescence bands at 753 and 813 nm are interpreted as originating from BPh (F-753) and BChl (F-813). Delayed emission signals between 700 and 800 nm were attributed to a metalloporphyrin phosphorescence. Surprisingly, the excitation spectra of the delayed BChl emissions measured at 872 and 912 nm showed the same excitation spectra as the short-wavelength emission, typical of metalloporphyrins. This is strong indication of an energy transfer between metalloporphyrins and BChl.