Abstract
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 CaCl(2), 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.
Highlights
The binding of cations (Na+, Ca++) to the phosphate group of lecithin showed the same order
Since the lecithin molecule possesses a phosphate and a trimethylammonium group separated by two methylene groups, its structure allows two ionic forms: one in which the separation of charges is maximal and the other in which a reduced separation of charges results from an internal salt linkage between the phosphate and trimethylammonium groups in the same molecule
The surface potentials of the mixed monolayers are the same on subsolutions of NaCl or CaC12, which indicates that the ionic phosphate group does not interact with calcium in the presence of neighboring trimethylammonium groups
Summary
Surface Pressure and Surface Potential Measurements The methods of measuring surface pressures by means of a modified Wilhelmy plate, and surface potentials by a radioactive electrode have been described previously [2]. Surface pressures and surface potentials of lecithin monolayers were measured on subsolutions of NaCl as well as CaClz (10-1~-10-%) a t pH 5.6 and 25OC. Surfacemeasurements were made on subsolutions of citric acidsodium citrate buffer (p = 0.05) a t p H 2-6, and TrisHC1 buffer (p = 0.05) a t pH 7-9. Inorganic chemicals of reagent grade and twice-distilledwater were used
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