Evaluation of the anesthetic-lipid association constant. A monolayer approach

A new approach is presented which allows to describe the binding of different local anesthetics to lipids. Lipids (DL- alpha-dipalmitoylphosphatidylcholine, phosphatidylserine, cardiolipin) are spread at the air-water interface and the anesthetic (procaine, butacaine, tetracaine) injected into the aqueous subphase. The equilibrium constants associated to the interfacial reaction: D+ (subphase) +L- (monolayer) in equilibrium DL (monolayer) (where D+ denotes the anesthetics, L- the lipid anionic site and DL the complex) are calculated from an experimental evaluation of the surface potential of the lipid monolayer. This mode of determination is based essentially on the good correlation between the experimental values of the surface potential and the theoretical predictions from the Gouy-Chapman theory. Fluorescence measurements on liposomes are carried out in order to locate the position of the drug in the lipid layer. This method can be extended to any positively charged drug-anionic lipid interaction.