INFLUENCE OF THE LENGTH OF THE SPACER ON THE PARTITIONING PROPERTIES OF AMPHIPHILIC FLUORESCENT MEMBRANE PROBES

Four fluorescent diphenylhexatriene derivatives were considered as mem brane probes, namely two ammonium compounds, 3-(diphenylhexatrienyl)pr opyltrimethylammonium (TMAP-DPH) and 22-(diphenylhexatrienyl)docosyltr imethylammonium (LcTMA-DPH), and two phospholipids, itoyl-2-[3-(diphen ylhexatrienyl)propanoyl]-sn-glyc ero-3-phosphocholine (DPHpPC) and iph enylhexatrienyl)henicosanoyl]-sn-phosphocholine (LcDPHpPC). For each p air, the molecules differ by the length of the polymethylenic spacer b etween the fluorescent moiety and the polar head, so one pair comprise s two short chain molecules (C3 spacer) and the other two long chain m olecules (C21 or C22 spacer). The partitioning of these probes between gel and liquid crystalline phases of multilamellar vesicles with bina ry composition (DEPC and DSPC) was measured by a method based on fluor escence anisotropy. The partitioning was shown to depend strongly on t he length of the spacer. Short chain probes preferably partition into fluid phases (K(f/s) = 1.7 +/- 0.3 for TMAP-DPH; 2.6 +/- 0.11 for DPHp PC), whereas long chain probes show a strong preferential partitioning for gel phases of the vesicles (K(f/s) = 0.12 +/- 0.06 for LcTMA-DPH; 0.22 +/- 0.11 for LcDPHpPC). This strong partitioning may be explaine d by the interdigitation of the long polymethylenic chains across the mid-point of the lipid bilayer (I.E. Mehlhorn et al. (1988) Biochim. B iophys. Acta 939, 151-159), which is enhanced by the better packing pr ovided by a gel phase.