Further evidence for closed, nonspherical aggregates in the cubic I(1) phase of lysolecithin and water.

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.