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

Effects of divalent cations (Ca2+, Mg2+, Ni2+ and Zn2+) on a zwitter-ionic phospholipid monolayer at the air-water interface are investigated by surface pressure-area isotherms and in situ X-rays scattering. Divalent cations lower the surface pressure for the fluid (LE) to condensed (L-2) phase transition in a strongly ion-specific manner. Surprisingly, the two-dimensional lattice dimensions and the tilt of the lipids' alkyl tails in the L-2 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 L-2 phase always displaying a tilt of 29 degrees 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 L-2 phase can be deduced at any surface pressure once the transition pressure is obtained from isotherms.