Molecular dynamics simulation of model lipid membranes: Structural effectsof impurities

The gel to fluid phase transition or ordered to disordered phase transition observed in biological membranes are simulated by using constant energy Mo lecular Dynamics. The surface part of the membrane is modelled as a two-dim ensional matrix formed by the head groups of the phospholipid molecules. He ad molecules which are modelled as three spheres fused with three force cen ters. interact with each other via van der Waals and Coulomb type interacti ons. The -so called- impurity or foreign molecule embedded in the surface r epresents the protein type molecule which is present in biological membrane s and control its activity. It is modelled as a pentagon having one force c enters in each corner. It also interacts with the surface molecules again v ia van der Waals and Coulomb type interactions. The surface density is kept constant in the simulations of the systems with or without impurity. Struc tural and orientational changes due to impurity were observed and proved by monitoring two-dimensional order parameter. It has been shown that melting of the surface or breakage of the ordering of the surface molecules become s easier and ordered to disordered phase transition temperature was lowered by 100 K if the impurity is present.