Further evidence for closed, nonspherical aggregates in the cubic I1 phase of lysolecithin and water

Abstract

Measurements of time-resolved fluorescence quenching have been performed in the binary lauroyllysophosphatidylcholine (LaLPC)/water system. The aggregation numbers, N, are determined for the micellar solution phase (N(micelle) approximately 80) and the cubic liquid crystalline I(1) phase (N(cub) approximately 90) at 298-303 K. When a quencher is present, the fluorescence decays for the hexagonal phase of the LaLPC/water system and for the bicontinuous cubic phase of monooleoylglycerol/water system are nonexponential, as expected for phase structures having long-range continuous apolar regions. Nuclear magnetic resonance (NMR) measurements of the lipid translational diffusion conclusively show that the cubic I(1) phase consists of closed micelles. NMR spectra of (31)P obtained at 202.4 MHz of this cubic phase exhibit a characteristic line shape, which is compatible with a phase structure containing short nonspherical micelles. A comparison between electron spin resonance (ESR) spin-label spectra recorded for a micellar solution and the cubic phases of the LaLPC and monooleoylglycerol systems are also shown to support a structure of closed micelles in the cubic I(1) phase of the lysolecithin system.

Ordered micellar and inverse micellar lyotropic phases

“…The best-known group of lipids that, in conjunction with water, will form ordered Type I micellar phases are the saturated lysophospholipids [62][63][64][65][66]. Although relatively scarce, with fewer than one phospholipid in 20 found in a typical cell being a lysophospholipid, they have been shown to play a major role in several signalling pathways, membrane fission and many other biological processes (see, e.g., [67,68]).…”

Cubic-Phase-Based Concentrated Emulsions

“…3 an excess-oil phase is present. It has been found (21,(24)(25)(26) that in some systems there is a close structural relation between the cubic phase and the adjacent micellar solution, and that the transition between these two phases looks very much like a melting process in a certain range of surfactant concentration. In fact, the nonfluid I 1 phase suddenly starts to flow at the boundary of the I 1 -phase region.…”

“…It is probable that oils stabilize the cubic phase by inducing the formation of micelles with a moderate axial ratio and, therefore, a more positive curvature (12,(25)(26)(27)(28)(29). This may improve the packing and order of micelles in the cubic structure so that the I 1 -W m phase transition temperature increases.…”

Lipid and water diffusion in bicontinuous cubic phases measured by NMR

“…In bicontinuous phases, the measured diffusion coefficients of the lipid and of the water are of the same order of magnitude as in lamellar phases and in pure water, respectively. On the other hand, in cubic phases built from closed aggregates, the apparent lipid diffusion coefficient is one or two orders of magnitude smaller than in the lamellar phase (Lindblom et al, 1976;Lindblom and Wennerstrom, 1977;Lindblom et al, 1979;Eriksson et al, 1982;Rilfors et al, 1986;Eriksson et al, 1987;Lindblom and Rilfors, 1989;Lindblom et al, 1992).…”