Fluorine-19 NMR spectroscopy of fluorinated analogs of tritrpticin highlights a distinct role for Tyr residues in antimicrobial peptides

Abstract

Because of their potential as novel antibiotic agents, antimicrobial peptides (AMPs) have generated considerable interest. The mechanism of bacterial toxicity of AMPs often involves the disruption and/or permeabilization of the bacterial membrane; even those that act intracellularly first have to traverse the membrane. In this work we have explored the incorporation of the fluorinated aromatic amino acids fluoro-Phe and fluoro-Tyr into the Trp- and Arg-rich AMP tritrpticin, and investigated their role in the membrane binding properties and the antimicrobial activity of the peptide. Fluorinated peptides were obtained with good yield by recombinant expression of tritrpticin as a calmodulin-fusion protein in Escherichia coli. Cells were grown in the presence of glyphosate, an inhibitor of aromatic amino acid biosynthesis, and the peptides were released by proteolysis from the purified fusion protein. By using SDS micelles, as a simplified model of the bacterial cytoplasmic membrane, we could study the peptide-membrane interactions and the preferred location of individual fluorinated residues in the micelles by 19F NMR spectroscopy. Solvent-perturbation 19F NMR measurements revealed that para-fluoro-Phe residues are embedded deeply in the hydrophobic region of the micelles. On the other hand, 3-fluoro-Tyr residues introduced in tritrpticin were located near the surface of the micelles with high solvent exposure, while 2-fluoro-Tyr sidechains were less solvent exposed. In combination with the outcome of determinations of their antimicrobial activity, our 19F NMR results indicate that the higher solvent exposure of Tyr residues correlates with a decrease of the antimicrobial potency. This different role of Tyr can likely be extended from tritrpticin to other cationic AMPs.