Paramagnetic isoprenoid carrier lipids. 2. Dispersion and dynamics in lipid membranes

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Abstract
The transbilayer diffusion rates and self-association of spin-labeled glycosyl carrier lipids and other isoprenoids were studied in model phospholipid membranes. Transposition rates of phosphorylated species in small (300 .ANG.) or large (.gtoreq. 1000 .ANG.) diameter unilamellar phosphatidylcholine (PC) vesicles are slow (t1/2 > 5 h at 25.degree. C); this argues against their proposed role in the transbilayer portage of sugar units during polysaccharide and glycoprotein assembly. The same probes mix well with several host lipids, remaining monomolecularly dispersed even at high ionic strength (0.5 M NaCl) or in the presence of various polyvalent cations. This behavior persists for several degrees beneath the transition temperature (Tc) of saturated lecithins. Neutral carboxylate analogs undergo pronounced reversible self-association. Aggregation is dependent on the probe concentration and nature of the host, but only weakly temperature-dependent above the Tc. Even at low probe concentrations (< 0.5 mol %) in fluid membranes, aggregates persist > 75.degree. C. Unsaturation in the lecithin fatty acyl chains dramatically increases isoprenoid monomer solubility. Segregation appears to involve relocation of the entire molecule in the membrane interior.