Bicellar mixtures containing pluronic F68: morphology and lateral diffusion from combined SANS and PFG NMR studies

Small angle neutron scattering (SANS) and pulsed field gradient (PFG) nuclear magnetic resonance (NMR) diffusion measurements were applied to examine morphology and diffusion in dimyristoyl- plus dihexanoyl-phosphatidylcholine bicellar mixtures, either neutral or negatively charged, incorporating a Pluronic triblock copolymer (F68). Negatively charged bicellar mixtures, doped with dimyristoylphosphatidylglycerol (DMPG), exhibited SANS profiles consistent with a perforated lamellar morphology for the magnetically alignable phase. Correspondingly, F68 diffusion in this magnetically aligned phase was normal Gaussian, in that the mean square displacements increased linearly with the experimental diffusion time, with a lateral diffusion coefficient of 1.9 x 10(-11) m(2) s(-1) consistent with a lipid bilayer inserted configuration. Neutral bicellar mixtures, that is, lacking DMPG, in contrast, displayed SANS profiles characteristic of ribbons arranged in such a fashion as to produce extended lamellae. Within the lamellae, the ribbons exhibited an in-plane periodicity (interribbon) of between 120 and 140 A. Correspondingly, F68 diffusion was non-Gaussian, exhibiting a square root diffusion time dependence of the mean square displacement indicative of one-dimensional curvilinear diffusion. The presence or absence of DMPG, rather than of F68, dictated the ribbon versus lamellar morphology, with F68 reflecting this difference via its lateral diffusion behavior. Although ribbons have been reported previously, this is the first study to show that they aggregate, most likely into extended lamellar sheets, and eventually fold into multilamellar vesicles.