1.O.1. The Voltage-induced Pore (ViP) Family
Formation of transient pores and the movement of anions and cations across lipid membranes have been studied by Kandasamy and Larson 2006 who performed systematic atomistic molecular dynamics simulations of palmitoyl-oleoyl-phosphatidylcholine (POPC) lipids. A double bilayer setup was employed, and different transmembrane potentials were generated by varying the anion (Cl-) and cation (Na+) concentrations in the two water compartments. A transmembrane potential of approximately 350 mV was thereby generated per bilayer for a unit charge imbalance. For transmembrane potential differences of up to approximately 1.4 V, the bilayers were stable over the time scale of the simulations (10-50 ns). At larger imposed potential differences, one of the two bilayers breaks down through formation of a water pore, leading to both anion and cation translocations through the pore. The anions typically have a short residence time inside the pore, while the cations show a wider range of residence times depending on whether they bind to a lipid molecule or not. Over the time scale of the simulations, Kandasamy and Larson 2006 did not observe the discharge of the entire potential difference, nor did they observe pore closure, although the size of the pore decreased as more ions translocated. They also observed a rare lipid flip-flop, in which a lipid molecules translocated from one bilayer leaflet to the opposite leaflet via the water pore.