Correlation of Three-dimensional Structures with the Antibacterial Activity of a Group of Peptides Designed Based on a Nontoxic Bacterial Membrane Anchor*

To understand the functional differences between a nontoxic membrane anchor corresponding to the N-terminal sequence of the Escherichia coli enzyme IIAGlc and a toxic antimicrobial peptide aurein 1.2 of similar sequence, a series of peptides was designed to bridge the gap between them. An alteration of a single residue of the membrane anchor converted it into an antibacterial peptide. Circular dichroism spectra indicate that all peptides are disordered in water but helical in micelles. Structures of the peptides were determined in membrane-mimetic micelles by solution NMR spectroscopy. The quality of the distance-based structures was improved by including backbone angle restraints derived from a set of chemical shifts (1Hα, 15N, 13Cα, and 13Cβ) from natural abundance two-dimensional heteronuclear correlated spectroscopy. Different from the membrane anchor, antibacterial peptides possess a broader and longer hydrophobic surface, allowing a deeper penetration into the membrane, as supported by intermolecular nuclear Overhauser effect cross-peaks between the peptide and short chain dioctanoyl phosphatidylglycerol. An attempt was made to correlate the NMR structures of these peptides with their antibacterial activity. The activity of this group of peptides does not correlate exactly with helicity, amphipathicity, charge, the number of charges, the size of the hydrophobic surface, or hydrophobic transfer free energy. However, a correlation is established between the peptide activity and membrane perturbation potential, which is defined by interfacial hydrophobic patches and basic residues in the case of cationic peptides. Indeed, 31P solid state NMR spectroscopy of lipid bilayers showed that the extent of lipid vesicle disruption by these peptides is proportional to their membrane perturbation potential.