X-ray studies on polymorphism of lamellar lecithin/local anaesthetic/water mixtures

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Abstract
Three mixtures with different compositions of 1,2-dipalmitoyl lecithin and the local anaesthetic heptacaine in dispersions with excess water, the so-called mixtures 36/1, 5/1 and 1/1, have been studied by X-ray diffraction techniques in the small and wide angle region as function of temperature. The polymorphism of mixture 36/1 and mixture 5/1 showed similarities. With increasing temperature the phase sequence Lp → Lβ0 → Lα0 was observed. In all phases the hydrocarbon chains of the bilayer are arranged in an orthogonal lattice. The low temperature gel phase Lp is characterized by herringbone chain packing. In the gel phase, the distances between the lamellae in the mixture 5/1 are increased by δd L of 1·4nm against mixture 36/1 and in the liquid-crystalline phase Lα 0 by δd L of 0·5 nm. In the Lp phase alterations in the distances between the lamellae are observed during the hysteresis of the phase transition ice ⇌ water. For the mixture 1/1 chain staggering was found in the low temperature phase, the gel phase and the liquid-crystalline phase.