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

摘要

A non-ideal lipid binary mixture (dilauroylphosphatidylcholine/ distearoylphosphatidylcholine), which exhibits get/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-l,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'-tetramethylindocarbocyanine (DiIC_(18)(3)), is usually reported in the literature as partitioning mainly to the gel. While intrinsic lifetime studies indeed indicated preferential partition of DiIC_(18)(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_(l8)(3) to the gel/fluid interphase. In order to fluidlocated donors sense these interphase-located acceptors, fluid domains should be small (not exceed ～ 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.