Interaction between phospholipid monolayer and volatile anesthetics using quartz crystal oscillator methods

The interaction between phospholipid monolayer (dihexadecyl phosphate: DHP) and volatile anesthetics (halothane) has been studied using quartz crystal microbalance (QCM) method and quartz crystal impedance (QCI) method. The quartz crystal oscillator was attached horizontally on the DHP monolayer that was formed on the water surface. At quite low halothane concentrations (≤4 mM), no change of frequency for QCM and resistance for QCI was observed. At low concentrations (6–13 mM), the frequency decreased and the resistance increased concomitant with the increased concentration. Both frequency and resistance approached saturation values asymptotically. The concentration at which the frequency changed (6 mM) differed from that at which the resistance changed (9 mM). At high concentrations (>14 mM), both frequency and resistance showed a sudden and discontinuous change (a frequency decrease and a resistance increase). These results indicate the following action mechanism of halothane on DHP monolayer: at low halothane concentrations, the halothane hydrates adsorb into the DHP monolayer/water interface. Subsequently, DHP monolayer/water interface reconstruction occurs because of the interaction and aggregation between adsorbed halothane hydrates with an increase in the hydrate on the interface, i.e., an increase in the DHP monolayer viscosity. At high concentrations, multilayer formation of halothane hydrates and/or promotion of the aggregation of halothane hydrates occur on the DHP monolayer/water interface. The viscosity of the interfacial layer (DHP monolayer and interfacial structured water) changes according to the action of halothane hydrates.