Studies of kinetics and equilibrium membrane binding of class A and class L model amphipathic peptides

We studied the kinetics and equilibrium membrane binding of two amphipathic alpha-helical peptides: the 18L peptide, which belongs to the class L (lytic peptides), and the Ac-18A-NH2 peptide of the class A (apolipoprotein), according to classification of Segrest et al. ((1990) Proteins, 8, 103-117). Both for cationic 18L and zwitterionic Ac-18A-NH2, the presence of acidic lipids increased the membrane binding constants by two orders of magnitude. The free energy of peptide-membrane association was in the range of 8.5-12.8 kcal/mol. Binding isotherms corresponded to monomer partitioning with saturation at high peptide/lipid ratios. This was also supported by stopped flow studies of the kinetics of peptide-membrane association as measured by peptide tryptophan fluorescence or by energy transfer from the peptide to the lipid-anchored anthrylvinyl fluorophor. The apparent time required for peptide-membrane equilibration was in the millisecond range. At low peptide/lipid ratios it depended on lipid concentration and was independent of the peptide concentration. The rate of peptide-membrane association was found to be relatively close to the diffusion limit. Thus peptide-membrane affinity was mostly determined by the peptide dissociation rate, i.e. higher membrane affinity correlated with a decrease in dissociation rate and with a slower peptide exchange. We have shown that the dynamic character of the peptide membrane equilibrium can be used for selective peptide targeting and disruption of membranes with a specific lipid composition.