Molecular dynamics simulation of oleic acid/oleate bilayers: an atomistic model for a ufasome membrane.

When oleic acid and oleate coexist in comparable amounts they form unilamellar vesicles called ufasomes in aqueous phase. Intrinsic pH sensitivity of ufasomes makes it an attractive vehicle for drug delivery. Physical properties of ufasomes have been studied by using spectroscopic techniques but an atomistic model for a ufasome has not been proposed. In this study molecular dynamics simulation was performed on oleic acid/oleate bilayers with the oleate concentration varying from 40 to 70 mol%. All the bilayers reached an equilibrium and stayed stable during a 40 ns simulation. Area per lipid increased with mol% of oleate probably due to charge repulsion between anionic oleate molecules. Oleate was pulled out toward the aqueous phase so that the carboxyl groups of oleic acid and oleate were separated by 0.392 nm in the bilayer of oleic acid/oleate 1:1. Water concentration at the depth of carboxyl group of oleate was five times as high as that of oleic acid. Number of hydrogen bonds between oleic acid and oleate was small in contrast to a proposal that it is an important factor for the bilayer stability. However there was an extensive array of hydrogen bonds between the lipids and water molecules. Acyl chain order was within a normal range for a lipid bilayer but lateral diffusion was an order of magnitude faster in oleic acid/oleate bilayer than in dioleoylphosphatidylcholine bilayer. Cholesterol increased the bilayer thickness and order parameter and decreased the rate of lateral diffusion.