Interaction of uranyl ions with phospholipid and cholesterol monolayers

Abstract

Surface pressures and potentials of egg and dipalmitoyl lecithins, dipalmitoyl phosphatidic acid, cholesterol, cholestanol, and eicosanyl trimethylammonium monolayers were measured on subsolutions of uranyl acetate and NaCl. Among these films only lecithin monolayers were influenced by the presence of uranyl acetate in the subsolution. It is evident from surface potential measurements that uranyl ions interact with phosphate groups in lecithin monolayers but not in phosphatidic acid monolayers. It is suggested that steric factors and ionic charges are involved in the interaction between phosphate groups and uranyl ions. The interaction of uranyl ions with the phosphate groups tends to solidify lecithin monolayers and to increase the surface potential. A comparison of surface pressure-area curves of dipalmitoyl lecithin and dipalmitoyl phosphatidic acid monolayers suggests that the trimethylammonium group causes a long-range repulsion up to a distance of 12 A in lecithin monolayers. Surface chemistry of various stains has been discussed in relation to electron microscopy of membranes.