THE INFLUENCE OF THE HYDRATION NUMBER IN A MIXED LIPID BILAYER WITH CHOLESTEROL

Abstract

The main structural component of biological membranes are phospholipids. A large proportion of the constituent lipids have monounsaturated hydrophobic tails. The study of this type of system is adequate for the development of a number of pharmaceutical substances. Molecular dynamics (MD) is suitable for describing the structure of membranes at the molecular level. The degree of hydration of the molecules is of essential importance for the correct composition of the atomistic models of the lipid bilayers. For this reason, an atomistic MD model of a lipid bilayer composed of 128 SOPC molecules and 128 cholesterol molecules symmetrically placed in both monolayers at a temperature of 273K was constructed. The Slipids force field was used, showing good results in the simulation of lipid systems. Three degrees of hydration corresponding to a hydration number (Hn) of 25, 40 and 50 were studied with two different water models (TIP3P and TIP4P). The interaction between the lipids and the aqueous phase is described by radial distribution function (RDF) and the number of hydrogen bonds. The basic parameters of the lipid bilayer related to the mobility of the heads such as lateral diffusion coefficient were calculated. It was found that the 25 hydration number resulted in an unrealistic immersion of the lipid heads in the water molecules. The high degree of hydration (50 waters per lipid) gives a better description of water, but there is no significant difference in surface phenomena compared to hydration number 40. The model containing 40 waters per lipid in combination with water model TIP3P reproduces experimental data and is suitable for further consideration.