Halogenation as a tool to tune antimicrobial activity of peptoids

Abstract

Antimicrobial peptides have attracted considerable interest as potential new class of antibiotics against multi-drug resistant bacteria. However, their therapeutic potential is limited, in part due to susceptibility towards enzymatic degradation and low bioavailability. Peptoids (oligomers of N-substituted glycines) demonstrate proteolytic stability and better bioavailability than corresponding peptides while in many cases retaining antibacterial activity. In this study, we synthesized a library of 36 peptoids containing fluorine, chlorine, bromine and iodine atoms, which vary by length and level of halogen substitution in position 4 of the phenyl rings. As we observed a clear correlation between halogenation of an inactive model peptoid and its increased antimicrobial activity, we designed chlorinated and brominated analogues of a known peptoid and its shorter counterpart. Short brominated analogues displayed up to 32-fold increase of the activity against S. aureus and 16- to 64-fold against E. coli and P. aeruginosa alongside reduced cytotoxicity. The biological effect of halogens seems to be linked to the relative hydrophobicity and self-assembly properties of the compounds. By small angle X-ray scattering (SAXS) we have demontrated how the self-assembled structures are dependent on the size of the halogen, degree of substitution and length of the peptoid, and correlated these features to their activity.

Figure 1

Structures of first-generation peptoids containing halogen atoms.

Figure 2

SAXS data showing the scattered intensity plotted towards the modulus of the scattering vector, q for 10-mer peptoids at 5 mg/mL obtained at BM29, ESRF at 37 °C. (A) displays the fully halogenated and (B) includes half-halogenated peptoids. The non-halogenated peptoid 19 is included in both graphs as a reference. Results indicate predominantly free unstructured peptoid chains with a small fraction of sheet-like filaments for compounds 19–21 and 24–27, and defined bundles for compounds 22 and 23, seen as an increase in the scattering intensity and a change in shape of the curve.

Figure 3

Full SAXS results for the fully iodinated peptoids measured with Bruker NANOSTAR instrumentation (except the 10-mer 23 measured at ESRF) in the indicated concentrations together with model fits (fitted using the bundle model as described in the supplementary information). Panels A, B, C and D shows the 6-mer (5), 8-mer (14), 10-mer (23) and 12-mer (32) respectively.

Figure 4

Chlorinated and brominated variants of peptoid 1 and shortened brominated peptoid 1 analogues.

Figure 5

IC₅₀ curves of the selected peptoids toward HaCaT cell line.