Solid-state nuclear magnetic resonance relaxation studies of the interaction mechanism of antimicrobial peptides with phospholipid bilayer membranes

An 18-residue peptide, KWGAKIKIGAKIKIGAKI-NH2 was designed to form amphiphilic P-sheet structures when bound to lipid bilayers. The peptide possesses high antimicrobial activity when compared to naturally occurring linear antimicrobial peptides, most of which adopt an amphipathic a-helical conformation upon binding to the lipids. The perturbation of the bilayer by the peptide was studied by static P-31 and H-2 solid-state NMR spectroscopy using POPC and POPG/POPC (3/1) bilayer membranes with sn-1 chain perdeuterated POPC and POPG as the isotopic labels. P-31 NMR powder spectra exhibited two components for POPG/POPC bilayers upon addition of the peptide but only a slight change in the line shape for POPC bilayers, indicating that the peptide selectively disrupted the membrane structure consisting of POPG lipids. H-2 NMR powder spectra indicated a reduction in the lipid chain order for POPC bilayers and no significant change in the ordering for POPG/POPC bilayers upon association of the peptide with the bilayers, suggesting that the peptide acts as a surface peptide in POPG/POPC bilayers. Relaxation rates are more sensitive to the motions of the membranes over a large range of time scales. Longer P-31 longitudinal relaxation times for both POPG and POPC in the presence of the peptide indicated a direct interaction between the peptide and the POPG/POPC bilayer membranes. P-31 longitudinal relaxation studies also suggested that the peptide prefers to interact with the POPG phospholipids. However, inversion-recovery H-2 NMR spectroscopic experiments demonstrated a change in the relaxation rate of the lipid acyl chains for both the POPC membranes and the POPG/POPC membranes upon interaction with the peptide. Transverse relaxation studies indicated an increase in the spectral density of the collective membrane motion caused by the interaction between the peptide and the POPG/POPC membrane. The experimental results demonstrate significant dynamic changes in the membrane in the presence of the antimicrobial peptide and support a carpet mechanism for the disruption of the membranes by the antimicrobial peptide.