Journal of Lipid Research, Vol. 8, 227-233, May 1967
Copyright © 1967 by Lipid Research, Inc.

The ionic structure of lecithin monolayers

Dinesh O. Shah and Jack H. Schulman

Stanley-Thompson Laboratory, School of Engineering, Columbia University, New York 10027

Surface potentials of mixed monolayers of dicetyl phosphate and eicosanyl trimethylammonium bromide (1:1) were the same on subsolutions of 0.02 m NaCl or 0.01 m CaCl₂, which indicated that ionic phosphate does not interact with Ca⁺⁺ in the presence of a neighboring trimethylammonium group. Surface potential-pH plots of dicetyl phosphate, and of dipalmitoyl, egg, and dioleoyl lecithins showed that as the pH of the subsolution is decreased the phosphate groups in the monolayer are neutralized in the order: dicetyl phosphate > dipalmitoyl lecithin > egg lecithin > dioleoyl lecithin.

The binding of cations (Na⁺, Ca⁺⁺) to the phosphate group of lecithin also showed the same order. The binding of Ca⁺⁺) to egg phosphatidic acid monolayers, as measured by the increase in surface potential, is considerably greater than that to egg lecithin.

These results suggest that there is an internal salt linkage between the phosphate and trimethylammonium groups on the same lecithin molecule. An increase in unsaturation of fatty acyl chains increases the intermolecular spacing, which reduces the ionic repulsion between polar groups, and hence strengthens the internal salt linkage. The results support the concept of a vertical rather than coplanar orientation of the phosphoryl choline group with respect to the interface. A position has been proposed for Ca⁺⁺ in the dipole lattice of lecithin from a consideration of the surface potential measurements.

Supplementary key words dicetyl phosphate • lecithin (dioleoyl, egg, dipalmitoyl) • surface potential-pH • surface potential-log (salt concn) • internal salt linkage • strength of the salt linkage • calcium binding • effect of cholesterol and unsaturation • position of calcium

Submitted on June 14, 1966
Revised on November 10, 1966
Accepted on January 24, 1967

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