LYOTROPIC AND THERMOTROPIC PHASE-BEHAVIOR OF HYDRATED MONOACYLGLYCEROLS - STRUCTURE CHARACTERIZATION OF MONOVACCENIN

This work is part of a long-term research effort to establish the rela tionship between lipid molecular structure and lyotropic and thermotro pic mesophase propensity by comparing the equilibrium temperature-comp osition phase diagrams of a homologous series of monoacylglycerols, He re we report the temperature-composition phase diagram of monovaccenin (a C18:1c11 monoacylglycerol) in water, constructed using small-angle X-ray scattering in the temperature range of ca. 0-110 degrees C and the composition range of ca. 0-60% (w/w) water in the heating directio n. The interpreted equilibrium phase diagram is based on several hundr ed discrete X-ray diffraction measurements in temperature-composition space recorded as a function of temperature in 5 degrees C increments and of composition in 4% (w/w) water increments on average. The phases identified and characterized structurally in this system include the lamellar crystal phase, the lamellar liquid crystal phase, two inverte d cubic phases (Q(230), Ia3d; Q(224), Pn3m), the inverted hexagonal ph ase, and the fluid isotropic phase. The monovaccenin/water phase diagr am is very similar to that of the monoolein (a C18:1c9 monoacylglycero l)/water system. However, there are important differences in transitio n temperatures and phase boundary positions between the two that we at tribute to differences in the effective length and shape of the corres ponding amphiphiles. The sensitive response to temperature and lipid i dentity of the average water channel radius in the fully hydrated, bic ontinuous cubic phase makes monoacylglycerol systems important potenti al candidates for controlled drug release and membrane protein crystal lization.