Characterization of the binding of the local anesthetics procaine and tetracaine to model membranes of phosphatidylethanolamine: A deuteron nuclear magnetic resonance study

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Abstract
The interaction of the local anesthetics tetracaine and procaine with multilamellar dispersions of phosphatidylethanolamine was investigated using 2H NMR of specifically deuterated anesthetics. Tetracaine partitioned more strongly than procaine into the lipid. The 2H NMR spectra showed a quadrupole doublet and a narrow line with the former corresponding to membrane-bound anesthetic and the latter to anesthetic free in solution. The integrated areas of the narrow line and of the doublet correspond to the concentrations of free and bound anesthetic predicted from the Kp [partition coefficient] values. There is no strong pH dependence for the quadrupole splittings of tetracaine, suggesting a similar depth of penetration into the lipid bilayer over the entire pH range. The data are consistent with a model in which tetracaine acts as a wedge to stabilize the phosphatidylethanolamine bilayer against transition to a hexagonal structure. Procaine is proposed to sit higher in the phosphatidylethanolamine bilayer than does tetracaine. The T1 [spin-lattice relaxation time] values were generally shorter in the membrane than in solution, suggesting slower motions, particularly for the aromatic ring of tetracaine.

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