MOLECULAR-DYNAMICS STUDY ON ELECTROSTATIC PROPERTIES OF A LIPID BILAYER - POLARIZATION, ELECTROSTATIC POTENTIAL, AND THE EFFECTS ON STRUCTURE AND DYNAMICS OF WATER NEAR THE INTERFACE

Electrostatic properties of the dipalmitoylphosphatidylcholine bilayer in water have been investigated on the basis of molecular dynamics ca lculations in the isothermal-isobaric ensemble. Analysis of the long-t ime trajectory of partial charges on the atoms showed that polarizatio n of the total system has an anisotropic fluctuation; the component al ong the bilayer normal was I order of magnitude smaller than that para llel to the bilayer plane, the polarization of lipid being canceled ou t mostly by that of water along the bilayer normal. A positive electro static potential was observed inside the membrane. The potential was f ound to vary in two steps across the interface; the first gradient com es from the preferential orientation of the ester group, and the secon d one is attributed to asymmetrical solvation of water around the phos phate group. Slower reorientation of the dipole projected on the bilay er normal was found than that on the bilayer plane. Dynamics of water near the headgroup was strongly influenced by the hydrogen bonding wit h the phosphate group.