Thermodynamic and kinetic studies on the association of melittin with a phospholipid bilayer

Association of the amphiphilic peptide melittin with unilamellar vesicles of dioleoylphosphatidylcholine has been experimentally investigated by means of circular dichroism, fluorescence energy transfer and stopped-flow experiments. Circular dichroism changes upon titration of the peptide with vesicles (at low salt concentration) were analyzed to yield thermodynamic association isotherms. These isotherms are quantitatively interpreted in terms of a monomer-monomer partitioning of melittin between the aqueous and bilayer media. The data can be very well fitted by theoretical curves based on a Gouy-Chapman surface potential. Energy transfer involving chemically modified tryptophan confirms a lack of aggregation of the associated peptide. According to the kinetic measurements the association proceeds in practice as a one-step process, which is rather fast but not fully diffusion-controlled. We propose a simple mechanism where the inherent conformational transition determines the overall rate.