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

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-nh₂) 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.