The Bending Elasticity of 1-Monoolein upon Relief of Packing Stress

Abstract

The bending elasticity of the monoglyceride 1-monoolein (MO) in water has been measured in the inverse hexagonal (HII) phase at 37 °C. At this temperature, fully hydrated MO is normally in an inverse bicontinuous cubic phase based on Schwarz's D surface, . The addition of either of the C23 chainlength hydrocarbons, tricosane or 9-cis-tricosene, at mole fractions with respect to MO in excess of 0.05, induces a phase transition into the HII phase. This transition is understood to occur because packing stresses in the hydrophobic regions of the HII phase are reduced to levels where this phase is at a lower free energy than the phase. We have used X-ray diffraction on gravimetrically prepared samples and samples subjected to an osmotic stress to determine the bending energy of MO in this phase. The evidence suggests that 9-cis-tricosene can relieve almost all of the packing stress in the HII phase. In this case, we find that the spontaneous radius of curvature at the pivotal surface, R 0, is −20.0 ± 0.3 Å, and the monolayer bending rigidity, κ, is (1.2 ± 0.1) × 10-20 J. Comparing these energetics with those of the phase indicates that the packing stress for an excess water, HII phase at 37 °C in the absence of 9-cis-tricosene would constitute at least 50% of the total equilibrium free energy. With tricosane, the packing stress cannot be completely relieved because tricosane melts at 47.6 °C in the bulk. The results from geometric measurements on this system suggest that when the packing stress is not fully relieved in the HII phase, the polar/apolar interface is deformed away from being cylindrical. Treating the interface as if it were cylindrical leads to a nonphysical location of the pivotal surface that is in disagreement with all previous measurements, a 35% increase in the magnitude of R 0, and a 4-fold increase in the calculated bending rigidity.

Tunable pDNA/DODAB:MO lipoplexes: The effect of incubation temperature on pDNA/DODAB:MO lipoplexes structure and transfection efficiency

“…In fact, temperature is known to dramatically influence the phase behavior, final curvature and fluidity of lipids in the absence of DNA [21,22], and those effects can be easily translated to the properties of the lipoplex itself. In the case of pDNA:DODAB:MO lipoplexes, where DODAB assumes the role of the bilayer-forming cationic lipid with an high melting temperature (T M = 45 • C) [23,24] and MO the role of the nonlamellar-forming neutral surfactant with a low melting temperature (T M = 32 • C) [25,26], as well as the molar fraction of both constituents is expected to strongly influence the physicochemical properties and structure of the final lipoplex, as previously observed in the absence of nucleic acids [27,28].…”

Development of Dioctadecyldimethylammonium Bromide/Monoolein Liposomes for Gene Delivery

“…MO is a natural-occurring neutral surfactant that has the particularity of forming two inverted bicontinuous cubic phases (QII D and QII G ) in excess water [61,62]. It possesses a single unsaturated acyl chain (C18:1) attached to a glycerol headgroup [63]. Its tendency to form inverted bicontinuous cubic phases has been explored in the past for different applications such as protein crystallization [64,65] or matrix for gel electrophoresis [66], and justifies the structural richness of the liposomal system formed with DODAB [40].…”

Absorption mechanism of DHP107, an oral paclitaxel formulation that forms a hydrated lipidic sponge phase

“…The driving force for the formation of the cubic phase in hydrated monoolein comes from the tendency of monoolein to curl away from the planar lamellar phase. This tendency can be expressed as the curvature elastic energy function [40,41] . Our findings, however, demonstrated that DHP107 did not form a cubic phase.…”