ALTERED ORGANIZATION OF LIGHT-HARVESTING COMPLEXES IN PHOSPHOLIPID-ENRICHED RHODOBACTER-SPHAEROIDES CHROMATOPHORES AS DETERMINED BY FLUORESCENCE YIELD AND SINGLET-SINGLET ANNIHILATION MEASUREMENTS
Whj. Westerhuis et al., ALTERED ORGANIZATION OF LIGHT-HARVESTING COMPLEXES IN PHOSPHOLIPID-ENRICHED RHODOBACTER-SPHAEROIDES CHROMATOPHORES AS DETERMINED BY FLUORESCENCE YIELD AND SINGLET-SINGLET ANNIHILATION MEASUREMENTS, Biochimica et biophysica acta. Bioenergetics, 1366(3), 1998, pp. 317-329
An improved method for fusion of liposomes to intracytoplasmic membran
e vesicles of Rhodobacter sphaeroides was developed that involves repe
ated cycles of freeze-thaw-sonication and provides a controlled proced
ure for phospholipid enrichment of up to 15-fold. In freeze-fracture r
eplicas, 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 w
as accompanied by reductions in energy transfer between the peripheral
LH2 and core LH1 antennae, as well as from LH1 to reaction centers. S
inglet-singlet annihilation at 4 K revealed a two-fold decrease in the
cluster size of core antenna BCh1s, which was also reflected by chang
es 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 cent
er complexes are associated, that most peripheral antenna complexes ar
e adjacent to at least one core assembly, and that fusion induces a se
paration of single LH1 and LH2 rings. At 4 K, a relatively large Stoke
s shift severely limits transfer between LH2 complexes in the native b
ilayer, while restricted transfer among two or three LH1 complexes ari
ses mainly from spectral inhomogeneity. This explanation also implies
that the anisotropic long-wavelength component of the LH1 absorption s
pectrum, which acts as an energy trap at 4 K, exists as an excitonic s
tate involving 6-8 BChls. (C) 1998 Published by Elsevier Science B.V.
All rights reserved.