Synthesis and Structural Characterization of All Four Diastereoisomers of the Crossed Alkene-Bridged Nisin DE-Ring Mimic

The lantibiotics represent a class of antimicrobial peptides, in which the unusual amino acids dehydroalanine and dehydrobutyrine and the intramolecular thioether bridges (lanthionines) are important structural features for bioactivity. Nisin is the most-prominent representative of the lantibiotics, and it inhibits the bacterial cell-wall biosynthesis by binding to lipid II via its N-terminus. The lipid II - nisin complex is responsible for pore-formation since the C-terminal part of nisin is inserted into the bacterial cell membrane which ultimately results in cell leakage and collapse of vital ion gradients. In order to increase the metabolic stability of nisin, the oxidation-sensitive thioether bridges can be replaced by metabolically stable dicarba moieties, as successfully demonstrated by the synthesis of nisin AB(C) analogs containing alkane/alkene bridges.[1] The most interesting fragment of nisin is the C-terminal intertwined DE-ring which has a i->i+3, i+2->i+5 connectivity pattern. To obtain more insight into the importance of this cross-bridged structure on nisin's bioactivity, we synthesized a series of all four diastereomers of the crossed alkene-bridged DE-ring mimic. This synthesis is based on the cyclization of a linear allylglycine-containing hexapeptide into the correctly knotted 1->4, 3->6 bicyclic hexapeptide using ring-closing metathesis, and all four diastereoisomers were obtained by HPLC and structurally characterized by NMR spectroscopy. An orthogonal protection scheme was used, to enable the independent N- or C-terminal modification of the bicyclic hexapeptides with azide/alkyne functionalities. Via Cu(I)-catalyzed cycloaddition chemistries, alkyne-functionalized natural ABC-fragments of nisin, which were obtained by tryptic digestion of full length nisin followed by HPLC purification, have been conjugated to synthetic DE-ring mimics to obtain novel nisin derivatives and their affinity toward lipid II and pore-forming capacity have been studied. Herein, we report on the details of the synthesis and characterization of the geometric isomers of the synthetic DE-ring mimics, and their use as synthons in Cu(I)-catalyzed click chemistry to obtain newly designed nisin hybrids as potential novel peptide antibiotics.