Thermodynamic and Biophysical Analysis of the Membrane-Association of a Histidine-Rich Peptide with Efficient Antimicrobial and Transfection Activities

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 acids through the cell membrane. We have investigated the membrane binding properties and energetics of LAH4-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-L1 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 ∼4 and ∼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.