Variable-pressure ²H NMR studies of bicellar mixtures in the presence and absence of DMPG

Rahmani, Ashkan (2013) Variable-pressure ²H NMR studies of bicellar mixtures in the presence and absence of DMPG. Masters thesis, Memorial University of Newfoundland.

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Abstract

Lipid dispersions with planar regions enriched in long chain lipids and highly curved regions enriched in short chain lipids can exhibit complex phase behaviour. Such "bicellar" mixtures provide insight into how the morphology of self-assembled soft structures are determined by component heterogeneity. Bicellar dispersion propert ies can be affected both by surface charge and the application of pressure. Variable pressure deuterium nuclear magnetic resonance (²H NMR) studies were carried out on bicellar lipid mixtures comprising t he short chain lipid 1,2-dihexanoyl- snglycero- 3-phosphocholine (DHPC), the long chain lipid 1 ,2-dimyristoyl-sn-glycero-3- phosphocholine (DMPC) and the anionic lipid 1,2-dimyristoyl-sn-glycero-3-phospho( 1'-rae-glycerol) (sodium salt) (DMPG). Samples of DMPC-d₅₄ / DMPG/ DHPC (3 :1:1 ), DMPC/DMPG-d₅₄ / DHPC (3: 1:1) and , for comparison, DMPC-d₅₄ / DHPC (4:1) were prepared and hydrated in HEPES buffer at lipid weight fractions of 10%. DMPCd₅₄ / DMPG/ DHPC (3:1:1) sample was also prepared with the same sample preparation protocol but the sonication and freeze-thaw omitted . Spectra and quadrupole echo decays were obtained, between 4⁰c and 65⁰c, for both warming and cooling at ambient pressure, 41.4 Mpa, and 82.7 Mpa. At ambient pressure, all mixtures form isotropically reorienting small particles at low temperature. On warming past the DlVIPC gel-to-liquid crystal transition temperature, the particles coalesce into larger structures that can be magnetically oriented. At higher temperatures, the dispersions form more randomly oriented lamellar phases. On cooling at ambient pressure, the lamellar phase persists to lower temperature where it transforms directly to the isotropic phase. Application of pressure raises the transition temperatures for DMPC-d₅₄ / DHPC (4:1) but does not significantly change the phases observed. For DMPC-d₅₄ / DMPG/DHPC (3:1:1), pressure can stabilize the isotropically reorienting phase to higher temperature on warming. On cooling under pressure, DMPCd₅₄ / DMPG/ DHPC (3:1:1) can reorganize into a highly ordered gel phase with spectra similar to those of the high pressure interdigitated gel phases of some phospholipids. The same phase behavior was observed for non-sonicated DMPC-d₅₄ / DlVIPG/DHPC (3:1:1) and for DMPC/ DMPG-d₅₄/DHPC (3:1:1). Once nucleated, the highly ordered gel phase appears to be stable even at ambient pressure. These observations demonstrate the sensitivity of dispersion morphology to details of mixed-lipid sample's thermal and pressure history.

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