Lipid–polyelectrolyte complexes at the air–water interface for different lipid packing

In presence of calcium ions in the water subphase, DNA and carboxymethyl cellulose (carboxyMC) can form complexes with the zwitterionic lipid dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) at the air–water interface. Depending upon the lipid packing, these polyelectrolytes can penetrate the lipid monolayer or can stay below the lipid monolayer after forming lipid–polyelectrolyte complexes, which are evidenced from the variation of the surface pressure with time at a constant area. Variation of surface pressure shows that at relatively less packing of DPPC molecules, (area per molecule is 119.5 Å2 in presence of DNA and 101.1 Å2 in presence of carboxyMC) surface pressure increases with time following the Langmuir growth relation. At intermediate packing, in presence of DNA, surface pressure decreases slowly compared to that of pure DPPC monolayer, whereas pressure remains nearly constant in presence of carboxyMC in the water subphase. At dense packing, i.e., at lower area per molecule (nearly 53 Å2 in presence of both DNA and carboxyMC), surface pressure decreases slowly for both DNA and carboxyMC, compared to that of pure DPPC monolayer. Mostly all the decay curves fits well with the stretched exponential function or with the sum of one constant and two exponential decay terms, indicating the existence of different reorganization processes of the lipid and such lipid-polyelectrolyte complexes at the air-water interface.