On the origin of multiphasic kinetics in peptide binding to phospholipid vesicles.

We critically examined a series of exact kinetic models for their ability to describe binding of a typical α-helical amphipathic peptide to lipid bilayers. Binding of the model peptide lysette-26 was measured through fluorescence resonance energy transfer from a Trp residue on the peptide to a fluorescently labeled acceptor lipid included in vesicles composed of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine. Experimental data was collected varying peptide and lipid concentrations over an order of magnitude. The kinetic models were fit to all the experimental data simultaneously. Of the four models examined, the simplest one that is sufficient to correctly describe the experimental data includes two coupled equilibria, one between peptide monomers in solution and bound to the lipid membrane, and a second one between lipid-bound peptides that oligomerize to form dimers. We found that individual kinetic binding curves are insufficient to distinguish among kinetic models of peptide binding to lipid bilayers but that a number of models can be excluded based on inspection of a simple set of experiments.