Article pubs.acs.org/JPCB
Thermodynamic and Biophysical Analysis of the MembraneAssociation of a Histidine-Rich Peptide with Efficient Antimicrobial and Transfection Activities Nataliia Voievoda,† Therese Schulthess,‡ Burkhard Bechinger,† and Joachim Seelig*,‡ †
Institute of Chemistry, University of Strasbourg/CNRS, UMR7177, 67070 Strasbourg, France Division of Biophysical Chemistry, Biozentrum of the University of Basel, Klingelbergstrasse 50/70, 4056 Basel, Switzerland
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S Supporting Information *
ABSTRACT: 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 physicalchemical methods. CD spectroscopy was employed to quantitate the membraneinduced 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.
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INTRODUCTION Linear cationic peptides such as the frog peptides PGLa and magainin1 show antimicrobial activities through the formation of membrane pores (reviewed in ref 2). Pore formation is facilitated by folding into amphipathic helices upon interaction with lipid membranes. In order to better understand the membrane interaction of such amphipathic sequences model peptides were synthesized using the natural sequences as templates.3 The so-called LAH4 peptides are composed essentially of alanine and leucine residues with four histidines placed strategically along the chain to allow a fold into a perfectly amphipathic α-helix. In addition, two lysines are located at the N-and C-termini to ensure a good solubility in water. The histidines have pKa values between 5.4 and 6.0 when measured in dodecylphosphocholine micelles.3,4 The overall hydrophobicity and the hydrophobic moment of the LAH4 molecule can be altered by merely changing the pH. At pH
