Dilute bicellar solutions for structural NMR work

Deuterium NMR spectroscopy has been employed to characterize the concentrat ion dependence of orientational order in DMPC/DHPC bicellar solutions with molar ratios q = [DMPC]/[DHPC] = 3.3, 2.7, and 2.3. The stability of a disc otic nematic phase can, in general, be predicted from a simple Onsager pict ure involving the size and concentration of the mesogenic unit, but for the bicellar solutions this model is not adequate. Specifically, macroscopic a lignment is observed at total lipid concentrations well below that, 1-10% ( w/w) predicted by Onsager's model. Thus the discotic nematic phase is stabl e to approximate to 3-5% (w/w) for q = 3.3-2.3, and the bicellar order is h ighest just before phase separation occurs at the minimum total phospholipi d concentration. This implies the presence of a DHPCbic reversible arrow DH PCsol equilibrium in establishing bicellar size, thereby extending the rang e of concentrations for which alignment occurs. Bicellar morphology has bee n verified for a wide range of concentrations, temperatures, and q-values, but as viscosity measurements demonstrate, major morphological changes take place as the temperature is reduced below 30 degrees C. (C) 1998 Academic Press.