Lipid flip-flopping. TMV and leaflet symmetry measurements of a POPE/POPG (3:1) double bilayer undergoing electroporation over 30 ns. A value of -2 in leaflet symmetry indicates a single flip-flop from the anodic to the cathodic leaflet. A pore was formed within 5 ns, which saw both POPE and POPG lipids from the anodic leaflet form the toroidal structure of the pore. After 15 ns, the POPE lipids within the pore return to the anodic leaflet while additional POPG lipids translocate to the cathodic leaflet. By 30 ns, one POPE lipid had flip-flopped from both leaflets (producing a symmetry of 0) and five POPG lipids had flip-flopped to the cathodic leaflet. This suggests that POPG lipids are more susceptible to flip-flopping towards the cathodic leaflet through transient water pores when under the influence of a TMV.