Cholesterol concentration effect on the bilayer properties and phase formation of niosome bilayers: A molecular dynamics simulation study

Niosome bilayers formed by the mixture of sorbitan monostearate (Span60) and cholesterol molecules with different cholesterol concentrations (0 to 70 mol% Chol) were studied by molecular dynamics simulations at the temperature of 298 K and the pressure of 1 bar. Structural properties and dynamics of niosome bilayers were characterized as a function of cholesterol concentration. We found that addition of cholesterol in the niosome bilayer significantly changes the molecular structure and stability. Increasing of cholesterol concentrations causes the area per molecule to increase and bilayer thickness to decrease, suggesting that the niosome bilayer is expanded from the gel to the liquid ordered phase. This contrasts with DOPC or DMPC lipid bilayers which show a condensation effect with increasing cholesterol content. The calculated isothermal area compressibility of the niosome bilayer shows a remarkable increase at moderate concentration (40–50 mol% Chol) and slight change at the higher concentrations. This suggests that the preparation of niosome with the higher cholesterol concentrations does not significantly improve the bilayer compressibility. With increasing cholesterol concentration the Span60 tails gradually adopt a more conformation while the orientation and dynamics of the Span60 groups are rather unaffected. The hydrogen bond interactions of the Span60/chol/water and Span60/water system increased with increasing cholesterol concentration, leading to improved bilayer stability.