NMR Studies of Micellar Aggregates in 1-Acyl-sn-glycerophosphocholine Systems. The Formation of a Cubic Liquid Crystalline Phase

Article abstract of DOI:10.1021/j100288a018

Measurements of the amphiphile diffusion coefficient, using the NMR pulsed-field gradient technique, have been performed on micellar solutions and cubic liquid crystalline phases of the following lysophosphatidylcholines: 1-lauroyl-sn-glycero-3-phosphocholine (LaLPC), 1-myristoyl-sn-glycero-3-phosphocholine (MyLPC), 1-palmitoyl-sn-glycero-3-phosphocholine (PaLPC), 1-stearoyl-sn-glycero-3-phosphocholine (StLPC), 1-oleoyl-sn-glycero-3-phosphocholine (OlLPC), and 1-linoleoyl-sn-glycero-3-phosphocholine (LiLPC).The 2H spin relaxation rates at two magnetic fields have been measured in micellar solutions of PaLPC.The phase equilibria at 25 and 35 deg C in the aqueous binary systems of LaLPC and MyLPC have been studied by 31P NMR and by optical techniques for lipid concentrations up to 50percent (w/w) 2H2O. 2H and 14N quadrupole splittings have been measured in anisotropic phases of PaLPC, OlLPC, and 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC).A cubic liquid crystalline phase, I₁, located between the micellar and hexagonal phase regions, is formed by LaLPC and MyLPC between approximately 40percent and 45percent (w/w) lipid in 2H2O.As previously shown the I₁ phase is also formed by PaLPC while not by StLPC, OlLPC, and LiLPC for which a concentrated micellar solution is in equilibrium with the hexagonal phase (Arvidson et al.Eur.J.Biochem. 1985, 152, 753).For LaLPC, amphiphile diffusion measurements show that the cubic phase I₁ consists of closed micellar aggregates.Amphiphile diffusion and 2H spin relaxation studies of the micellar phases show that the aggregates formed by LaLPC, MyLPC, PaLPC, and StLPC remain small and globular over the whole micellar phase region, while the micelles of OlLPC and LiLPC are large and polydispersed.Micelle diffusion measurements provide information about aggregate interactions, which is shown to be compatible with magnitudes of the hydration force and the van der Waals interactions previously obtained for corresponding bilayers.The formation of the cubic face I₁ is discussed in terms of the packing of interacting micellar aggregates in a newly proposed structure for the cubic phase.