Donor-donor energy migration (DDEM) as a tool for studying aggregation in lipid phases

A BODIPY(R)-labelled sulfatide (N-(BODIPY(R)- FL-pentanoyl) -galactosylcere broside-sulfate, hereafter abbreviated as BD-Sulfatide) was solubilised at different concentrations in lipid vesicles of 1,2-dioleoyl-sn -glycero-3-ph osphocholine (DOPC). Time-correlated single photon counting experiments sho w that the fluorescence relaxation is mono-exponential (with a lifetime of 6.5 ns) at molar ratios of BD-Sulfatide: DOPC that are less than 1:100. The fluorescence steady-state anisotropy decreases monotonously at molar ratio s smaller than 1:1000, which is compatible with donor-donor energy migratio n (DDEM) among the BODIPY(R) groups. A model that assumes DDEM across the l ipid bilayers, as well as in their planes, was used to analyse the time-res olved fluorescence anisotropy. Only two parameters appear in the model name ly; the bilayer thickness (d) and the average number density (C-2) distribu tion of BD-Sulfatide in the lipid bilayers. The extracted d-values vary bet ween 35 and 40 Angstrom, which is about the reported thickness of a bilayer of DOPC (38 Angstrom). Hence, the BODIPY(R) groups are preferentially loca ted in the water-lipid interface. At low concentration the experimental C-2 -values and those independently calculated are in good agreement, while the experimental values gradually become lower with increasing BD-Sulfatide co ncentration. These results are compatible with an aggregation of the sulfat ides and self-quenching of BODIPY(R), which is clearly established at highe r concentrations of the BD-Sulfatide. (C) 2001 Elsevier Science B.V. All ri ghts reserved.