Local anesthetics induce interdigitation and thermotropic changes in dipalmitoylphosphatidylcholine bilayers

The molecular mechanism underlying the action of local anesthetics is still elusive. Phenylethanol (PEtOH) is an ingredient of essential oils with a rose-like odor and has been used as a local anesthetic. In this work, we have explored the effect of PEtOH on thermotropic behavior and organization of dipalmitoylphosphatidylcholine (DPPC) membranes utilizing differential scanning calorimetry (DSC) and small angle X-ray scattering (SAXS). Our results indicate that the phase transition temperature of DPPC exhibited decrease with increasing PEtOH concentration. This is accompanied by hysteresis (difference in phase transition between the heating and cooling scans). We defined the threshold concentration of PEtOH as the concentration at which the difference in phase transition temperature between the heating and cooling thermograms is maximum. Interestingly, changes in enthalpy, entropy, and full width at half maximum displayed biphasic behavior beyond the threshold concentration of PEtOH. The biphasic change in thermodynamic parameters corresponding to phase transtition, coupled with hysteresis, is indicative of interdigitation in DPPC bilayers. We confirmed this proposition by SAXS measurements which show formation of the interdigitated phase in DPPC bilayers at and above the threshold concentration of PEtOH. To the best of our knowledge, these results constitute the first report describing the interdigitation of membrane bilayers induced by PEtOH. We further show that the formation of interdigitated phase in DPPC bilayers depends on PEtOH concentration and temperature. Our results could be useful in ongoing efforts to address the mechanism of action of local anesthetics in model and biological membranes.