Sequence Inversion and Phenylalanine Surrogates at the β-Turn Enhance the Antibiotic Activity of Gramicidin S

Abstract

A series of gramicidin S (GS) analogs have been synthesized where the Phe (i+1) and Pro (i+2) residues of the β-turn have been swapped while the respective chiralities (D-, L-) at each position are preserved, and Phe is replaced by surrogates with aromatic side chains of diverse size, orientation and flexibility. Although most analogs preserve the β-sheet structure, as assessed by NMR, their antibiotic activities turn out to be highly dependent on the bulkiness and spatial arrangement of the aromatic side chain. Significant increases in microbicidal potency against both Gram-positive and Gram-negative pathogens are observed for several analogs, resulting in improved therapeutic profiles. Data indicate that seemingly minor replacements at the GS β-turn can have significant impact on antibiotic activity, highlighting this region as a hot spot for modulating GS plasticity and activity.

Antimicrobial Electrospun Fibers of Polyester Loaded with Engineered Cyclic Gramicidin Analogues

“…In ethanol, the three peptides exhibit secondary structures (Figure 2a). More specifically, the spectrum of GA-S2 was interpreted as a combination of β-sheet (83%), β-turn (9%), and disordered (8%) motives, which was fully consistent with the reported 2D NOESY spectra recorded in H2O/D2O 9:1 v/v [36]. In contrast, GA-S1 and GA-L exhibited a combination of regular and distorted α-helical structures with a small fraction of disordered motives (< 5%).…”

“…Furthermore, reported results indicated a higher activity for GA-S2 against Gram-positive bacteria. The lower activity of GA-S1 was attributed to the absence of an aromatic moiety in the Ac3c residue (Scheme 1), which destabilized the bioactive conformation of the GA-S parent peptide [36]. Although the antimicrobial behavior of PE44 fibers loaded with GA-S1 and GA-S2 is limited by the peptide release rate, the activities displayed in Figure 9 are in good agreement with those reported for the peptides in solution [36].…”

“…The polymer has a melt flow index of 1.6 g/10 min (measured at 190 • C under a load of 2.16 kg according to ASTM-D1238). Linear GA was purchased from Sigma-Aldrich (G5002, St. Luis, MO, USA), while GA-S1 and GA-S2 were prepared as previously reported [36].…”

“…As it can be seen, PE44/GA-S1 and, especially, PE44/GA-S2 inhibit the growth of Gram-negative bacteria, whereas unloaded PE44 and PE44/GA-L fibers are harmless to such bacteria. It is worth noting that GA-L is active against Gram-positive bacteria and against a few selected Gram-negative microbial (e.g., Neisseria bacteria) [36], explaining the inactivity displayed in Figure 9a. In contrast, GA-S is indistinctly effective against some Gram-positive and Gram-negative bacteria.…”

“…Among the different strategies proposed to protect peptides from proteolytic cleavage, targeted replacements with noncoded amino acids are among the most successful ones [33][34][35]. In an early study, Cativiela and coworkers designed and synthetized two rigid GA analogues by introducing some chemical changes at the sequence of gramicidin soviet (GA-S) to enhance the antimicrobial activity of GA by stabilizing the bioactive conformations through steric and stereochemical constraints [36]. In addition, the incorporation of non-proteinogenic residues into GA-S should also impart stability against proteolytic cleavage.…”