LAH4-L1 is a synthetic amphipathic peptide with- antimicrobial activity: The sequence of the 23 amino acid peptide was inspired by naturally occurring :frog peptides such as PGLa and magainin. LAH4-L1 also facilitates :the transport Of nucleic adds through the cell membrane: We have investigated the membrane binding properties and energeties of LAH-L1 at pH 5:5 With physical-chemical methods. CD spectroscopy was employed to quantitate the membrane-induced random coil-to-helix transition of LAH4-L1. Binding isotherms were obtained, with CD spectroscopy as a function of the lipid-to-protein ratio for neutral and negatively charged membranes and were analyzed with both the Langmuir multisite adsorption model and the surface partition/Gouy Chapman model. According to the Langmuir adsorption model each molecule LAH4-L1 binds 4 POPS molecules, independent of the POPS concentration in the membrane This is supported by the surface partition/Gouy Chapman model which predicts an electric charge of LAH4-1,1 of z = 4. Binding affinity is dominated by electrostatic attraction. The thermodynamics Of the binding process was elucidated with isothermal titration calorimetry. The ITC data revealed that the binding process is composed-of at least three different reactions, that is, a coil-to-helix transition with an exothermic enthalpy of about -11 kcal/mol and two endothermic processes with enthalpies of similar to 4 and similar to 8 kCal/mol, respectively, which partly compensate the exothermic enthalpy of the Conformational change. The major endothermic reaction is interpreted as a deprotonation reaction following the insertion of a highly charged Cationic peptide into a nonpolar environment.