Binding of Short Cationic Peptides (KX)₄K to Negatively Charged DPPG Monolayers: Competition between Electrostatic and Hydrophobic Interactions

Abstract

The influence of the peptide sequence on the binding of short cationic peptides composed of five lysines alternating with uncharged amino acids within the series (KX)₄K to negatively charged monolayers of 1,2-dipalmitoyl-<i>sn</i>-glycero-3-phosphoglycerol (DPPG) was investigated by adsorption experiments in combination with epifluorescence microscopy. To evaluate the impact of electrostatic and hydrophobic contributions, different uncharged amino acids X with increasing hydrophobicity, where X = G (glycine), A (alanine), Abu (α-aminobutyric acid), V (valine), or L (leucine) were introduced into the peptide sequence to tune the peptide hydrophobicity. The adsorption kinetics of these peptides to a DPPG monolayer always showed two superimposed processes, one leading to an increase and another to a decrease of the surface pressure Π. Thus, the plots of the change in Π after peptide binding vs initial surface pressure of the monolayer showed an unusual behavior with maxima and negative changes in Π at high initial Π values. Epifluorescence microscopy confirmed that electrostatic binding of the peptides with a concomitant decrease in Π leads to a condensation of the lipid monolayer and the formation of liquid-condensed (<i>LC</i>) domains even at Π values where the monolayer is supposedly in the liquid-expanded (<i>LE</i>) state. An increase in hydrophobicity of the amino acid X was found to counteract the condensation and an increase in Π upon peptide binding is observed at low Π values, also concomitant with the formation of <i>LC</i>-domains. Compression of monolayers after peptide adsorption at low surface pressure for 4 h leads to a change of the isotherms compared to pure DPPG isotherms. The phase transition of DPPG from <i>LE</i> to <i>LC</i> state is smeared out or is shifted to higher surface pressure. Considerable changes in the shapes of <i>LC</i>-domains were observed after peptide binding. Growth of the <i>LC</i>-domains was hindered in most cases and regular domain patterns were formed. Binding of (KL)₄K leads to a decrease in line tension and the formation of extended filaments protruding from initially circular domains