Altered organization of light-harvesting complexes in phospholipid-enriched Rhodobacter sphaeroides chromatophores as determined by fluorescence yield and singlet-singlet annihilation measurements

Abstract

An improved method for fusion of liposomes to intracytoplasmic membrane vesicles of Rhodobacter sphaeroides was developed that involves repeated cycles of freeze-thaw-sonication and provides a controlled procedure for phospholipid enrichment of up to 15-fold. In freeze-fracture replicas, the fusion products appeared as closed vesicles of increased size and reduced intramembrane particle densities. Fluorescence yield measurements at 300 and 4 K showed that the gradual bilayer dilution was accompanied by reductions in energy transfer between the peripheral LH2 and core LH1 antennae, as well as from LH1 to reaction centers. Singlet-singlet annihilation at 4 K revealed a two-fold decrease in the cluster size of core antenna BChls, which was also reflected by changes in fluorescence polarization spectra. Energy transfer dynamics and structural considerations suggested that the annihilation curves were affected by non-uniformities. When taken into account, this led to the conclusion that in native membranes, on average two LH1-reaction center complexes are associated, that most peripheral antenna complexes are adjacent to at least one core assembly, and that fusion induces a separation of single LH1 and LH2 rings. At 4 K, a relatively large Stokes shift severely limits transfer between LH2 complexes in the native bilayer, while restricted transfer among two or three LH1 complexes arises mainly from spectral inhomogeneity. This explanation also implies that the anisotropic long-wavelength component of the LH1 absorption spectrum, which acts as an energy trap at 4 K, exists as an excitonic state involving 6–8 BChls.