The influence of local anesthetics on molecular organization in phosphatidylethanolamine membranes

The influence of the local anesthetics tetracaine (TTC) and procaine (PRC) on bilayers of specifically deuterated phosphatidylethanolamines (PE) has been studied by 2H and 31P NMR. Dimyristoylphosphatidylethanolamines (DMPE), deuterated at positions 2, 4, and 14 of the sn-2 chain, position 2 of the sn-1 chain, and in the ethanolamine headgroup, were mixed 1:1 with a semisynthetic egg PE and the effect of measured quantities of TTC and PRC on the 2H quadrupole splittings, spin-lattice relaxation times, and 31P chemical shift anisotropy were observed. Experiments were performed at pH 5.5, when the anesthetics are primarily charged, and at pH 9.5, when they are uncharged. Tetracaine was observed to disorder the hydrocarbon region of the bilayer and to induce a conformational change in the PE headgroup. Conversely, procaine had little or no effect on the hydrocarbon region and induced only a small change in the headgroup. These conformational changes and disordering effects, when adjusted for anesthetic partitioning, are essentially independent of the charge on the anesthetic. However, at pH 5.5 and low TTC/PE molar ratios (less than 0.1), the 2H NMR spectra showed two lipid environments--one corresponding to free PE and the other to PE in contact with TTC. Continued addition of TTC resulted in the eventual disappearance of the free PE signal and the corresponding growth of the signal from PE in contact with TTC. At pH 9.5, when TTC is uncharged, only one signal is observed. This indicates that at low pH, when TTC is primarily charged, it has a much slower rate of lateral diffusion in the PE bilayer. In mixtures of PE and phosphatidylserine, a conformational change in the headgroup was noted which was similar to that seen in the pure PE; however, there was no evidence for slow lateral diffusion of the anesthetics. The effects of TTC and PRC on the PE bilayer, when combined with our earlier study of the labeled anesthetics [Kelusky, E.C., and I.C.P. Smith, Biochemistry, 22:6011-6017 (1983)], indicate that TTC penetrates into the hydrocarbon portion of the bilayer whereas PRC sites only in the headgroup region.