Partitioning of uncharged local anesthetic benzocaine into model biomembranes

The partitioning of uncharged local anesthetic benzocaine (BzC) into molecular aggregates formed by cationic surfactant decylammonium chloride (DeAC) and phospholipid dipalmitoylphosphatidylcholine (DPPC) was studied from the surface tension and light transmittance measurements. The quantities concerning the partitioning of BzC, the compositions of BzC in the surface-adsorbed film and micelle and three kinds of differential partition coefficients corresponding to phase transitions of the DPPC bilayer membrane were evaluated from thermodynamic analysis of the experimental data. The surface-adsorbed film and micelle were more abundant in BzC than the aqueous solution and significantly large differential partition coefficients for the DPPC membranes were observed. The results clearly showed that the BzC molecules greatly partitioned into hydrophobic environments produced by surfactant-monolayer and phospholipid-bilayer membranes. The partitioning behavior of BzC was also compared with that of charged local anesthetic procaine hydrochloride (PC.HCl). It was shown that the PC.HCl molecule did not or hardly partition into such hydrophobic environments. The contrasting results of the partitioning between BzC and PC.HCl are attributable to the drastic decrease of hydrophilicity of BzC due to the lacking of ionic polar head group in comparison with PC.HCl.