Langmuir zyxwvu 1991, 7, zyxwvu 964-974 Interaction of Procaine with Stearic Acid Monolayers at the Air/Water Interface Maria Tomoaia-CotigelfJ and D. Allan Cadenhead'ts Department of Physical Chemistry, University of Marburg, zyxwv 0-3550 Marburg, FRG, and Department of Chemistry, State University of New York at Buffalo, Buffalo, New York 14214 Received May 21, 1990. In Final Form: October 19, 1990 The adsorption characteristics of procaine have typically been studied by measuring the surface tension of aqueous procaine solutions and by recording compressional isotherms, i.e., surface pressure as a function of molecular area, for stearic acid monolayers in the presence of various procaine subphase concentrations at both pH 2 and pH 8. The presence of the stearic acid monolayers promotes the enhanced adsorption of procaine in liquid or fluid states of the stearic acid monolayers. On compression of the stearic acid monolayer the procaine adsorption increases and, after attaining a maximum value, decreasesand vanishes near the collapse of the stearic acid monolayer. The surface characteristics derived from the compressional isotherms, as well as the area increase recorded at constant surface pressure, are interpreted by taking into account models both for the mechanism of procaine penetration into stearic acid monolayers and for the protolytic equilibria occurring in the system. The penetration numbers, i.e., the ratio of procaine molecules to stearic acid molecules in the mixed penetrated films, are derived from the molecular area incrementa and are in good agreement with the values obtained by using the Gibbs' adsorption equation. Molecular species of procaine are initially thought to be adsorbed at the air/water interface in a horizontal orientation. On compression of the spread stearic acid film, the procaine is gradually forced to adopt a vertical position. At higher surface pressure values procaine is squeezed out from the monolayer and is accumulated in an adjacent layer, thus causing a significant increase in the collapse pressure of the stearic acid monolayers. At pH 2 this latter effect has been interpreted in terms of ion-dipole interactions between the positively charged procaine molecules and the uncharged carboxyl groups of the stearic acid monolayers and in terms of hydrogen bonding between the carboxyl group and primary amino group of the procaine monocation. At pH 8 electrostatic interactions between the stearate anions and the procaine monocationare also taken into account. Procaine penetration occurred preferentially in the liquid or fluid phase of stearic acid monolayers and was found to be dependent on the surface pressure. Maximum penetration was established a t around 10 mN/m for monolayers on pH 2 and at 5 mN/m for stearic acid monolayers at pH 8. The maximum values are much higher at pH 8 than at pH 2 due to the protolytic equilibria in which both the stearic acid and the procaine participate. The physical insertion of procaine between the stearic acid molecules is presumably primarily responsible for the expansion effect found. At the sametime, electrostaticinteractionsor ion-dipoleinteractions zyxw will modify such interchain interactions, although the latter are increasingly important in the solid state of the monolayer as collapse is approached. Introduction Local anesthetics are known to exert their action by closing the sodium channels' of nerve membranes, thus blocking nerve signal propagation. The molecular mech- anism of anesthetic action has been the subject of extensive studies, but it is still unclear whether this blocking is the result of a direct anesthetic-protein interaction2 or a perturbation by anesthetic of the lipid matrix changing some physicochemical property of the bilayer surrounding the channel^.^ Studies performed using different experimental tech- niques showed that some anesthetics increase the fluidity of lipid bilayers4and/ or decrease the order in hydrocarbon chains- and either increase the surface area of mono- + University of Marburg. t Permanent address: Department of Physical Chemistry, Uni- 1 State University of New York at Buffalo. (1) Trudell, J. R. In Molecular Mechanisms zyxwvutsr of Anesthesia: Progress in Anesthesiology; Fink, B. R., Ed.; Raven Press: New York, 1980: Vol. versity of Cluj-Napoca, 3400 Cluj-Napoca, Romania. 2, pp 261-270. (2) Bogga, J. M.; Roth, S. M.; Yoong, T.; Wong, E.; Hsia, J. C. Mol. (3) Seeman, P. hog. Anesthesiol. 1975, I, 243-251. (4) Trudell, J. R.; Hubbell, W. L.: Cohen, E. N. Fed. Proc. 1972.31. Pharmacol. 1976,12, 136-143. 549. (5) Turner, G. L.; Oldfield, E. Nature (London) 1979,277,669-670. (6) Boulnnger, Y.; Schreier, S.; Smith, I. C. P. Biochemistry 1981,20, (7) Kelusky, E. C.; Smith, I. C. P. Biochemistry 1983,22,6011-6017. 68244830. layers maintained at constant surface pressureloor increase the surface pressure of lipid films maintained at constant area.11 Although these experiments involve different physical parameters, the molecular origin of the effects observed is presumably the same, viz. the modification of the structural and dynamic properties of ordered acyl chains due to the binding and penetration of anesthetics into oriented lipid systems. However, there is also the very real possibility that various anesthetics act by different mechanisms. The quantification of the interaction with, or the binding to and penetration of anesthetics into, a membrane model system is clearly a fundamental problem in biophysics. Very few studies of the effects of tertiary amine anesthetics such as procaine on membrane models have been carried out, and their interpretation at a molecular level is not always completely obvious. Thus, it appears that procaine is bound by egg phosphatidylethanolaine~ but is only weakly bound by the more ordered acyl chains in identical acyl chain egg phosphatidylcholine8 aqueous lamellar dispersions. While in this case, the differing results must be attributed to the differing polar head- (8) Kelusky, E. C.; Smith, I. C. P. Can. J. Biochem. Cell Biol. 1984, (9) Auger,M., Jarrell,H. C.,Smith, I. C.P.,Siminovitch,D. J., Mnntach, (IO) Seelig, A. Biochim. Biophys. Acta 1987,699, 196-204. (11) Vilallonga, F. A.; Phillips, E. W. J. Pharm. Sci. 1979,6C, 314-316. 62, 178-184. H. H.; Wong, P. T. T. Biochemistry 1988,27,6086-6093. , ~-, 0 1991 American Chemical Society