Metal complexation with Langmuir monolayers of histidyl peptide lipids

Langmuir monolayers made from peptide-lipid molecules represent a novel direction in the research areas of biomimetic interfaces and two-dimensional supramolecular chemistry. Peptide structures and molecular recognition activities toward other guest molecules have been the focus of previous study. This study reports the investigation of metal complexation to histidine-containing peptide lipids in the organized Langmuir, Langmuir-Schaefer, or Langmuir-Blodgett films. Three peptide lipids PEP1-PEP3, with a histidine amino acid incorporated in the middle of the peptide, were designed and synthesized. The monolayer structures and metal-binding activities of each peptide lipid and their 1:1:1 molar ratio mixture were studied by thermodynamic and spectroscopic techniques. It was found that hard Lewis acid type metal cations such as K+ and Mg2+, and borderline or soft metal cations such as Zn2+, Cu2+, and Cd2+ exhibit clearly different binding activity toward peptide-lipid monolayers. The conformational changes of peptides upon binding with Cu2+ and Zn2+ were partially revealed by FT-IR spectroscopic studies. Furthermore, by adding a fluorescent-probe lipid to the peptide monolayer, dramatic fluorescence change was observed when Cu2+ or Zn2+ bound to the Langmuir and Langmuir-Schaefer films of peptide-lipid monolayers. Metal-protein complexation plays a crucial role in the function and activity of proteins and enzymes. Investigation of metal complexation to organized peptide Langmuir monolayers may provide an alternative approach for the development of artificial metalloproteins and novel supramolecular systems or materials.