Effects of Divalent Cations on Phase Behavior and Structure of a Zwitterionic Phospholipid (DMPC) Monolayer at the Air−Water Interface

Abstract

Effects of divalent cations (Ca2+, Mg2+, Ni2+, and Zn2+) on a zwitterionic phospholipid monolayer at the air−water interface are investigated by surface pressure−area isotherms and in situ X-ray scattering. Divalent cations lower the surface pressure for the fluid (LE) to condensed (L2) phase transition in a strongly ion-specific manner. Surprisingly, the two-dimensional lattice dimensions and the tilt of the lipids’ alkyl tails in the L2 phase show a nearly ion-nonspecific dependence on the excess surface pressure above the transition pressure. An empirical “universal” relationship was found between the tail tilt and the excess pressure, with the tails in the L2 phase always displaying a tilt of 29° at the transition. A practical implication of these results is that, regardless of the divalent cation present, the microscopic details of the lipid tail packing in the L2 phase can be deduced at any surface pressure once the transition pressure is obtained from isotherms.