Calorimetric studies of the thermodynamic properties of lamellar lecithin/local anaesthetic/water mixtures

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Abstract
Differential scanning calorimetry has been used to study the effects of two different local anaesthetics from heptacaine type (abbreviated HK and IR-1) on the thermodynamic properties of dipalmitoyl lecithin in bilayers. The model membrane systems studied were multilamellar aqueous dispersions of unbranched and branched dipalmitoyl lecithin and their mixtures in the presence of the local anaesthetics. The calorimetric results show that both anaesthetics differ qualitatively in their miscibility effects. Incorporation of the anaesthetic HK into the bilayer from aqueous dipalmitoyl lecithin dispersions causes a large decrease of the main transition temperature (transition gel phase → Lα, phase). For the anaesthetic IR-1, this decrease in the main transition temperature is considerably smaller. In accord with the phase diagram of the ternary system dipalmitoyl lecithin/ heptacaine/water, demixing takes place in the low temperature phase at a mixing ratio n r of about 16/1 molecules of lecithin per molecule of heptacaine HK. However, if the anaesthetic IR-1 is used instead of HK, demixing in the gel phase does not occur until the mixing ratio has reached the value 1/1 molecule of lecithin per molecule of IR-1. Furthermore heptacaine effects the miscibility behaviour of the ternary system unbranched lecithin/branched lecithin/water. The miscibility gap within the gel phase is extended but the type of phase diagram is not changed.

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