Spectral and functional comparisons between the carotenoids of the two antenna complexes of Rhodopseudomonas capsulata

The spectral and functional properties of carotenoids associated with each of the two light-harvesting complexes of the Rhodopseudomonas capsulata photosynthetic antenna system have been distinguished by studying mutants lacking one or the other complex. In mutants containing only the light-harvesting I complex (LH-I), the absorption spectrum of the carotenoids is blue-shifted compared to wild type. Carotenoid absorption in mutants possessing only the light-harvesing II complex (LH-II) complex is red-shifted. The circular dichroism spectrum of carotenoids in each complex is also distinctive. Although carotenoids in each complex function with approximately the same efficiency in harvesting and transmitting light energy for photosynethesis, only the carotenoids associated with LH-II undergo an electrochromic bandshift upon generation of a transmembrane potential. These observations are interpreted to indicate that both the orientation of carotenoid molecules with respect to the plane of the membrane, and the immediate electrochemical environment of these molecules differ in the two light-harvesting complexes.