The thermodynamics of binding of the antibacterial peptide magainin 2 amide (M2a) to negatively charged small (SUVs) and large (LUVs) unilamellar vesicles has been studied with isothermal titration calorimetry (ITC) and CD spectroscopy at 45 degrees C. The binding isotherms as well as the ability of the peptide to permeabilize membranes were found to be qualitatively and quantitatively similar for both model membranes. The binding isotherms could be described with a surface partition equilibrium where the surface concentration of the peptide immediately above the plane of binding was calculated with the Gouy-Chapman theory. The standard free energy of binding was deltaG0 approximately -22 kJ/mol and was almost identical for LUVs and SUVs. However, the standard enthalpy and entropy of binding were distinctly higher for LUVs (deltaH0 = -15.1 kJ/mol, deltaS0 = 24.7 J/molK) than for SUVs (deltaH0 = -38.5 kJ/mol, deltaS0 = -55.3 J/molK). This enthalpy-entropy compensation mechanism is explained by differences in the lipid packing. The cohesive forces between lipid molecules are larger in well-packed LUVs and incorporation of M2a leads to a stronger disruption of cohesive forces and to a larger increase in the lipid flexibility than peptide incorporation into the more disordered SUVs. At 45 degrees C the peptide easily translocates from the outer to the inner monolayer as judged from the simulation of the ITC curves.