Bicellar Mixtures Containing Pluronic F68: Morphology and Lateral Diffusion from Combined SANS and PFG NMR Studies

Abstract

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.

Temperature Driven Annealing of Perforations in Bicellar Model Membranes

“…84 However, when such bicellar mixtures contain the negatively charged phospholipid DMPG, a swollen lamellar phase is observed by SANS. 78 Such a lamellar phase is also observed upon adding small amounts of lanthanide ions, such as Tm 3þ or Yb 3þ ,whichbindwithhighaffinity to the phosphates of the constituent phospholipids.…”

Applications of neutron and X-ray scattering to the study of biologically relevant model membranes

“…The ribbons transformed into MLVs with increasing temperature, while the charged perforated lamellae remained unaltered. In the case of high total lipid concentration (≥20 wt.%) systems, the lamellar phase is comprised of entangled ribbons, as was recently confirmed using a combination of SANS and pulsed field gradient NMR studies (Soong et al, 2010). It should be pointed out that bilayered disks were identified by SANS to exist at low temperature , where the NMR signal was found to be isotropic.…”

In-situ temperature-controllable shear flow device for neutron scattering measurement—An example of aligned bicellar mixtures

“…[39][40][41] The low-T bicelles are not alignable, while the high-T lamellae are alignable under a shear flow with their bilayer normal perpendicular to the confined surfaces. 33 More interestingly, the aligned structure remains even several hours after the flow ceases.…”

“…42 The bicelle-to-lamellae transition has also been reported elsewhere. 39 To evaluate the degree of alignment of the sample under the shear flow, "rocking curve" measurement was performed by fixing the detector angle at the 1st order Bragg peak position (i.e., 2θ B ) and collecting the scattering intensity as a function of sample angle, θ. The attainable θ range was between −40 • and 40 • lest that the supporting posts might block the incident beam as descried in "Instrumental Design" section.…”