Amphiphatic α-helices are the lipid-binding motif in many apolipoproteins. Two model peptides, namely Asp-Trp-Leu-Lys-Ala-Phe-Tyr-Asp- Lys-Val-Ala-Glu-Lys-Leu-Lys-Glu-Ala-Phe (18A) and Lys-Trp-Leu-Asp-Ala-Phe- Tyr-Lys-Asp-Val-Ala-Lys-Glu-Leu-Glu-Lys-Ala-Phe (18R), have been synthesized previously to mimic the structural and functional properties of apolipoprotein A-1. Here a quantitative thermodynamic analysis of the binding process of 18A and 18R to neutral and negatively charged lipid membranes is provided. Peptide 18A has a higher lipid affinity than 18R, and both peptides bind better to mixed 1-palmitoyl-2-oleoyl-3-sn-glycero-phosphocholine/1- palmitoyl-2-oleoyl-3-sn-glycero-phosphoglycerol (POPC/POPG) bilayers than to pure POPC bilayers. At lipid-to-peptide ratios >100, the binding of 18A and 18R to phospholipid bilayers can be described by an apparent surface partition equilibrium with binding constants in the range of 40-900 M-1. At high peptide concentrations, the membrane affinity of 18A and 18R increases dramatically. NMR studies provide evidence that peptide-peptide interactions make additional contributions to the binding energy. A cooperative binding model is developed to describe the binding process over the whole concentration range. The cooperativity parameter σ is identical for 18A and 18R yielding a peptide-peptide interaction energy of about -2.4 kcal/mol. The free energy of membrane insertion is about -6.5 kcal/mol for 18A and -5.5 kcal/mol for 18R. The binding reaction is driven by the hydrophobic surface energy which is partially balanced by the loss in translational and rotational degrees of freedom. A molecular analysis of the free energy of binding predicts a 40-60% insertion of the peptides into the hydrophobic membrane environment. Deuterium and phosphorus solid state NMR were used to monitor the influence of 18A and 18R on the long range and short range order of the phospholipids. The sprectra are characteristic of fluid-like lipid bilayers and provide no evidence for the formation of discoidal particles. However, both peptides change the conformation of the phosphocholine dipoles, moving the N+ end of the latter toward the water phase. The rotation of the -P-N+ dipoles is due to the interaction of the phospholipids with the positive charges on 18A and 18R, with 18A being more effective than 18R. For 18R the NMR data predict a pK shift and a partial charge neutralization of the carboxylate groups located at the edge of the polar/nonpolar interface.