Computer simulations of the electric interactions between the phospholipidhead-groups and ionic admixtures in the membrane surface

Some phospholipids (e.g. lecithin) form a system of electric dipoles on the membrane surface layer. In the case of lecithin the positive dipole charge is located on the choline and the negative one on the phosphoric molecule group. These dipoles are arranged almost parallel to the membrane surface. Taking the dipole membrane structure as a base for further investigations, a computer model of the electrostatic interaction between the dipole system and the ionic admixture was investigated. The model presumes hexagonal cen tered or a rectangular flat geometry of the 121 dipoles distribution. The d ipoles may rotate freely around round the motionless symmetry axis perpendi cular to the system surface. The initial state is given by fixing the geome try of the dipole matrix and ionic admixture distribution. Subsequently thi s system underwent a computer simulation which consisted of a calculation o f resultant force moments acting on each dipole caused by other dipoles and ions. These force moments lead the system to the equilibrium state (minimu m of the binding energy). The minimum energy val;e of the dipoles system de pends on concentration and charge of the admixtured ions. The results of re peated simulations indicate that the system achieve the least of all bindin g energy (the most stable equilibrium state) at 1.5% concentration of admix tured ions in case of ion charge equal to 1Q (where Q denotes arbitrary uni t of ion charge) and at 2.5% concentration of admixtured ions in case of io n charge equal to 2Q. The calculated results are in a good agreement with t he experimental.