INTERACTIONS OF LARGE IONS WITH PHOSPHOLIPID MONOLAYERS

Surface pressure-area isotherms of dimyristoylphosphatidylcholine, dipalmitoylphosphatidylcholine, dimyristoylphosphatidylethanolamine, and dipalmitoylphosphatidylethanolamine monolayers have been measured at 25°C on buffer solutions from pH 1.5 to 12.5, and the effects of phosphotungstate ions and of uranyl ions in the subphase have been determined. Phosphotungstate causes expansion of the monolayers at low pH values, indicating some penetration into the monolayer region and an associated increase in the bulk phase pH for protonation of the phosphate. Uranyl ions cause contraction of expanded monolayers at low pH, which is attributed to adsorption of the U022+ drawing neighboring phospholipid molecules together. Hydrolysis of the uranyl ions at higher pH values increases their size and reduces their charge, leading to decreased effects on the monolayers. It is concluded that the use of phosphotungstic acid or uranyl acetate as a “stain” for electron microscopy could lead to significant distortions in membrane or vesicle structure. © 1990, American Chemical Society. All rights reserved.