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

Abstract

Three mixtures with different compositions of 1,Zdipalmitoyl 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 511 showed similarities. With increasing temperature the phase sequence L, + Lq --* L, was observed. In all phases the hydrocarbon chains of the bilayer are arranged in an orthogonal lattice. The low temperature gel phase L, is characterized by herringbone chain packing. In the gel phase, the distances between the lamellae in the mixture S/l are increased by Ad, of 1.4nm against mixture 36/1 and in the liquid-crystalline phase L, by Ad, of 0.5 nm. In the L, 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. IntroductionThe preceding paper [l] dealt with results from calorimetric measurements on the influence of two local anaesthetics of the heptacaine type on the phase and miscibility behaviour of the ternary and quaternary dipalmitoyl lecithin/water systems. The thermodynamic analysis of the phase diagram in the ternary lecithin/anaesthetic/ water mixtures indicates that the two local anaesthetics differ qualitatively in their miscibility properties in aqueous lecithin dispersions. These calorimetric investigations are now supplemented by X-ray diffraction measurements on three selected mixtures of heptacaine.The X-ray diffraction investigations were carried out in aqueous dispersions with different mixtures of the following composition: mixture 36/1, this means 36 molecules of dipalmitoyl lecithin per molecule of heptacaine; mixture 5/1, has 5 molecules of dipalmitoyl lecithin per molecule of heptacaine and mixture 1/1, has 1 molecule