Biophysical Journal 63: 723-729 (1992)
© 1992 the Biophysical Society

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

Department of Physical Chemistry, University of Umeå, S-90187 Umeå, Sweden
Department of Physiological Chemistry, University of Uppsala, Biomedical Center, S-751 23 Uppsala, Sweden

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 80) and the cubic liquid crystalline I₁ phase (N_cub 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 I₁ phase consists of closed micelles. NMR spectra of ³¹P 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 I₁ phase of the lysolecithin system.