Partition of membrane probes in a gel/fluid two-component lipid system: a fluorescence resonance energy transfer study

A non-ideal lipid binary mixture (dilauroyiphosphatidylcholine/distearoylphosphatidylcholine), which exhibits gel/fluid phase coexistence for wide temperature and composition ranges, was studied using photophysical techniques, namely fluorescence anisotropy, lifetime and resonance energy transfer (FRET) measurements. The FRET donor, N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)-dilauroylphosphatidylethanolamine, and a short-tailed FRET acceptor, 1,1'-didodecil-3,3,3',3'-tetra-methylindocarbocyanine (DiIC(12)(3)), were shown to prefer the fluid phase by both intrinsic anisotropy, lifetime and FRET measurements, in agreement with published reports. The other studied FRET acceptor, long-tailed probe 1,1'-dioctadecil-3,3,3',3'-tetra-methylindocarbocyanine (DiIC(18)(3)), is usually reported in the literature as partitioning mainly to the gel. While intrinsic lifetime studies indeed indicated preferential partition of DiIC18(3) into a rigidified environment, FRET analysis pointed to an increased donor-acceptor proximity as a consequence of phase separation. These apparently conflicting results were rationalized on the basis of segregation of DiIC(18)(3) to the gel/fluid interphase. In order to fluid/located donors sense these interphase-located accepters, fluid domains should be small (not exceed similar to 10-15 nm). It is concluded that: membrane probes which apparently prefer the gel phase may indeed show a non-random distribution in this medium, and tend to locate in an environment which simultaneously leads to less strict packing constraints and to favorable hydrophobic matching interactions. (C) 2000 Elsevier Science B.V. All rights reserved.