Site of action of the local anesthetic tetracaine in a phosphatidylcholine bilayer with incorporated cardiolipin.

Tetracaine (TTC) increases the permeability of phospholipid liposomal membranes to water, and this increase is reduced by the incorporation of cardiolipin into the membranes. We examined the molecular interaction of a phospholipid with the TTC cation in egg-yolk phosphatidylcholine (EyPC) liposomal membranes with incorporated bovine heart cardiolipin (BhCL) by IR spectroscopy and by determination of partitioning and the pKa of membrane-bound TTC. The IR spectra indicated that TTC shifted the stretching band of the BhCL PO2- group, a potential site of hydration in the bilayer, to a lower frequency but did not shift that of EyPC. TTC intercalated into the BhCL bilayer shifted its aromatic C-N stretching band to a lower frequency. One molecule of TTC was found to bind approximately five molecules of EyPC, and the incorporation of negatively charged BhCL into EyPC membranes increased the degree of binding of TTC to the bilayer membranes. The pKa values of TTC bound to membranes were determined as 7.7, 9.4, and 10.2 for EyPC membranes, EyPC membranes containing 50 mol % BhCL, and BhCL membranes, respectively, whereas that in an aqueous 10-mM NaCl solution was 8.5, as it was dependent on the manner of binding. The IR data together with the partitioning and the pKa data suggested differences between the actions of the TTC cation on negatively charged BhCL and on neutrally charged EyPC polar groups in the region close to the aqueous interface of the lipid bilayer.