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

Abstract

A BODIPY®-labelled sulfatide ( N-(BODIPY®-FL-pentanoyl)-galactosylcerebroside-sulfate, hereafter abbreviated as BD-Sulfatide) was solubilised at different concentrations in lipid vesicles of 1,2-dioleoyl- sn-glycero-3-phosphocholine (DOPC). Time-correlated single photon counting experiments show 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 ratios smaller than 1:1000, which is compatible with donor–donor energy migration (DDEM) among the BODIPY® groups. A model that assumes DDEM across the lipid bilayers, as well as in their planes, was used to analyse the time-resolved fluorescence anisotropy. Only two parameters appear in the model namely; the bilayer thickness ( d) and the average number density ( C 2) distribution of BD-Sulfatide in the lipid bilayers. The extracted d-values vary between 35 and 40 Å, which is about the reported thickness of a bilayer of DOPC (38 Å). Hence, the BODIPY® groups are preferentially located 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 concentration. These results are compatible with an aggregation of the sulfatides and self-quenching of BODIPY®, which is clearly established at higher concentrations of the BD-Sulfatide.