ACYL CHAIN ORIENTATIONAL ORDER IN LARGE UNILAMELLAR VESICLES - COMPARISON WITH MULTILAMELLAR LIPOSOMES - A H-2 AND P-31 NUCLEAR-MAGNETIC-RESONANCE STUDY

Large unilamellar vesicles (LUVs) composed of 1-[H-2(31)]palmitoyl-2-o leoyl phosphatidylcholine (POPC-d31), with diameters of approximately 117 +/- 31 and 180 +/- 44 nm, were prepared by extrusion through polyc arbonate filters with pore sizes of 0.1 and 0.2 mum, respectively. The 2H nuclear magnetic resonance (NMR) spectra obtained at 21-degrees-C contain two components: a broad component (approximately 17 kHz linewi dth) corresponding to the methylene groups and a narrower component or iginating from the methyl groups. Spectra with increasing powder patte rn characteristics were obtained by reducing the rate of phospholipid reorientations by addition of glycerol (to increase the solvent viscos ity) and by lowering the temperature. Full powder spectra, characteris tic of liquid-crystalline bilayers, were obtained for both LUV samples at 0-degrees-C in the presence of 50 wt% glycerol. Individual quadrup olar splittings were not resolved in these spectra, due to broader lin ewidths in the LUVs, which have significantly shorter values for spin- spin relaxation time T2 measured from the decay of the quadrupolar ech o (90 mus) than the multilamellar vesicles (MLVs; 540,us). Smoothed or der parameter profiles (OPPs) were obtained for these samples by integ ration of the dePaked spectra. The OPPs were very similar to the OPP o f POPC-d-31 MLVs in 50 wt% glycerol at the same temperature, indicatin g that orientational order in MLVs and LUVs with a diameter of greater -than-or-equal-to 100 nm is essentially the same. The presence of 80 w t% glycerol was found to have a disordering effect on the vesicles.