The effects of external fields on the structure of lipid bilayers.

1. A statistical mechanical model is proposed for describing the phase transition of phospholipid bilayers induced both by temperature and by hydrostatic pressure. 2. The calculated properties of phase transition, e.g. the transition temperature and pressure, the Clapeyron slope, the changes of enthalpy, of volume, of surface area, of thickness and of gauche number per chain, are comparable with available experimental data for dipalmitoylphosphatidylcholine (DPPC) dispersions. 3. Having determined the vertical Young's modulus of the bilayer, we have concluded that, in the case of pure lecithin membranes, the electromechanical compressive effect of electric field is negligible (the bilayer is compressed by less than one percent) up to 6 V. 4. It is pointed out that the average number and radius of instable pores in pure lecithin membranes increase drastically with increasing transmembrane potential from 150 mV. 5. The pore model can explain the mechanical rupture of pure lecithin membrane and predicts a drastic increase of conductivity near 200 mV.