Low-temperature fluorescence and absorption spectroscopy of reaction center/antenna complexes from Ectothiorhodospira mobilis, Rhodopseudomonas palustris and Rhodobacter sphaeroides

Abstract

Reaction centers (RC's) with either one or both light-harvesting complexes attached (RC/LH 1- or RC/LH 1/LH II complexes), have been isolated from the photosynthetic purple bacteria Ectothiorhodospira mobilis, Rhodopsculomonas palustris and Rhodobacter sphaeroides, using ionic and non-ionic detergents. The isolated complexes were analyzed with respect to their physical and functional properties using low-temperature (77 K) absorption, circular dichroism and fluorescence spectroscopy. The stability of the association between the antenna complexes and the RC, and consequently the functional interaction of the RC/antenna complexes after isolation, differed among the bacteria investigated in this study, particularly with respect to detergent sensitivity. By using the non-ionic detergent octyl glucoside we were able to isolate functionally intact RC/antenna complexes from E. mobilis, while for the isolation of these complexes from Rps. palustris and Rb. sphaeroides the ionic detergent cholate had to be used. In the RC/antenna complexes from E. mobilis, the energy transfer between LH II and the core antenna was sensitive to detergents, as judged by an increase of the LH II fluorescence at 77 K. A strong increase in the LH 1 and LH II fluorescence in RC/antenna complexes isolated with octyl glucoside from Rb. sphaeroides indicated that in this organism the functional interaction between both LH II and LH 1, as well as between LH 1 and the RC was easily disrupted. In Rps. palustris, the functional association between the antenna complexes and the RC was stronger compared to the other two investigated species.