Clustering Zwitterionic Amino Acids at the Upper Rim of Cone Calix[4]arene Triggers the Selective Recognition of Gram-Negative Bacterial Envelope

Eleven calix[4]arene ligands, bearing zwitterionic alpha-amino acids or charged ammonium or sulfonate/carboxylate groups, are synthesized and screened for the binding to the envelopes of three bacterial strain representatives of Gram-positive, Gram-negative, and mycobacteria. The binding is followed by on-cell Saturation Transfer Difference Nuclear Magnetic Resonance (NMR) experiments directly on alive cells. While the anionic tetrasulfonatocalixarene does not bind to any bacterial strains significantly and the cationic calixarenes strongly bind to both Gram-positive and Gram-negative bacteria, the zwitterionic tetraprolino- and tetraphenylalaninocalix[4]arene show a remarkable selectivity for Gram-negatives over Gram-positives and mycobacteria. The tetraprolinocalixarene binds to the lipopolysaccharides extracted from two Gram-negative bacteria (Pseudomonas putida or Escherichia coli), suggesting these biomacromolecules as possible targets in the recognition of their cell walls. The ligand binding epitope map demonstrates a deep involvement of the amino acids and calixarene aromatic nuclei in the interaction. In this study, for the first time, the ability of synthetic macrocycles to selectively recognize the envelope of Gram-negative bacteria is highlighted, and the way to the chemical modifications of the ligand structure is paved to develop devices for the detection or treatment of bacterial infections, thus allowing to add another string to the bow for the fight against antimicrobial resistance.