Effective charge of melittin upon interaction with POPC vesicles.

The binding of bee venom melittin to small unilamellar vesicles and large nonsonicated multilamellar bilayer membranes composed of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) was studied by means of circular dichroism, 31P-NMR and electrophoretic mobility. The melittin binding isotherm for small unilamellar vesicles (SUV) could be described by a partition equilibrium with Kp = (6 +/- 1).10(4) M-1. Electrostatic effects were taken into account by means of the Gouy-Chapman theory. Combining the partition equilibrium with the Gouy-Chapman analysis suggested an effective charge for melittin of Zp = 1.9, which is lower than the true electric charge of 5-6. The variation of the 31P-NMR signal of SUV showed the change in potential at the phosphodiester moiety of the lipid upon addition of melittin. This potential change was lower than that for an ion with an electrical charge of 5-6 and corresponded to a charge of 1.5. Electrophoretic mobility measurements with multilamellar vesicles confirmed the charge reduction effect. These experimental results show that the use of the simple Gouy-Chapman theory requires an effective electrical charge of the melittin which is lower than the formal charge.