MODELING OF THE ELECTROLYTE ION PHOSPHOLIPID LAYER INTERACTION

The self-consistent anisotropic field theory for chain molecules in in homogeneous systems has been applied to the analysis of ionic behavior at the lipid-water interface of free standing phospholipid bilayers a nd surface-adsorbed phospholipid monolayers. Fundamental in the theory is that the conformations of lipid molecules are generated with a rot ational isomeric state approximation on a lattice and weighted accordi ng to Boltzmann statistics where the local self-consistent field poten tial is computed using Flory-Huggins chi-parameters and averaged conta ct fractions. Electrostatic energies are also incorporated (in a Poiss on-Boltzmann way) into the model so that quite complex molecules, in t his case zwitterionic and charged phospholipids, are considered. Resul ts show that lipid head group P-N orientations in phosphatidylcholine (PC) monolayers and bilayers are angled from the layer plane in two pr obable conformations, whereas the head groups of phosphatidylserine (P S) in PS monolayers and bilayers have only one preferred conformation which is tilted toward the solution. The interaction of potentially pe rmeant cations with the lipid was studied in the presence of screening electrolyte which is present at about 103 times the permeant ions' vo lume fraction in solution. Because of the form of the potential profil e in both PC and PS layers, bulk electrolyte cations associate electro statically with the phosphate group in both lipids and to a lesser ext ent with the carboxyl group in PS and themselves modify the potential profile across the lipid layer. The attraction of cations to the polar groups is greater with negatively charged PS than with zwitterionic P C and increases with the charge of the cation. Although there is a lar ge difference between the potential profiles across monolayers of PC a nd PS in a monovalent cation 1:1 electrolyte, the difference in potent ial profiles across the two lipid layers PC and PS respectively is not so great in 2:1 and 3:1 electrolytes. This is reflected in the adsorp tion of permeant cations at the lipid-water interface.