Epimers l- and d-Phenylseptin: How the relative stereochemistry affects the peptide-membrane interactions.

Abstract In recent decades, several epimers of peptides containing d -amino acids have been identified in antimicrobial sequences, a feature which has been associated with post-translational modification. Generally, d -isomers present similar or inferior antimicrobial activity, only surpassing their epimers in resistance to peptidases. The naturally occurring l -Phenylseptin ( l -Phes) and d -Phenylseptin ( d -Phes) peptides (FFFDTLKNLAGKVIGALT- nh 2) were reported with d -epimer showing higher activity against Staphylococcus aureus and Xanthomonas axonopodis in comparison with the l -epimer. In this study, we combine structural (CD, solution NMR), orientational (solid-state NMR) and biophysical (ITC, DSC and DLS) studies to understand the role of the d -phenylalanine in the increase of the antimicrobial activity. Although both peptides are structurally similar in the helical region ranging from D4 to the C-terminus, significant structural differences were observed near the peptides' N-termini (which encompasses the FFF motif). Specific aromatic interactions involving the phenylalanine side chains of d -Phes is responsible to maintaining the F1-F3 residues on the hydrophobic face of the peptide, increasing its amphipathicity when compared to the l -epimer. The higher capability of d -Phes to exert an efficient anchoring in the hydrophobic core of the phospholipid bilayer indicates a pivotal role of the N-terminus in enhancing the interaction between the d -peptide and the membrane interface in relation to its epimer.