Dilute bicellar solutions for structural NMR work

Abstract

Deuterium NMR spectroscopy has been employed to characterize the concentration 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 discotic nematic phase can, in general, be predicted from a simple Onsager picture involving the size and concentration of the mesogenic unit, but for the bicellar solutions this model is not adequate. Specifically, macroscopic alignment is observed at total lipid concentrations well below that, 1-10% (w/w) predicted by Onsager's model. Thus the discotic nematic phase is stable to approximately 3-5% (w/w) for q = 3.3-2.3, and the bicellar order is highest just before phase separation occurs at the minimum total phospholipid concentration. This implies the presence of a DHPCbic -->/<-- DHPCsol equilibrium in establishing bicellar size, thereby extending the range of concentrations for which alignment occurs. Bicellar morphology has been 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 degreesC.