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Herein the synthesis of two nisin AB dicarba analogs is described, focusing on amino acid modifications at positions 2 and 5. The nisin mimics were synthesized by a combination of solid phase synthesis of the linear peptides, followed by macrocyclization via ring-closing metathesis and fragment assembly by means of solution phase chemistry. The two N-terminal nisin AB-fragment mimics contain either the native dehydrobutyrine (Dhb)/dehydroalanine (Dha) amino acid residues or alanine at position 2 and 5, respectively. The native dehydrobutyrine at position 2 and dehydroalanine at position 5 were introduced as their precursors, namely threonine and serine, respectively, and subsequent dehydration was carried out by EDCI/CuCl as the condensing agent. Both AB-fragment mimics were analyzed in a lipid II binding assay and it was found that the Ala2/Ala5 AB-mimic (2) showed a reduced activity, while the Dhb2/Dha5 AB-mimic (3) was as active as the native AB-fragment (1).

The antimicrobial peptide nisin is a promising template for designing novel peptide-based antibiotics to improve its drug-like properties. First steps in that direction represent the synthesis of hybrid nisin derivatives that contain a native nisin ABC-part and synthesized cross-stapled DE-ring fragments and are described here. The biological activity of the newly synthesized nisin derivatives was evaluated in order to compare the bioactivity of the synthetic DE-ring containing mimic and native lanthionine-bridged DE-ring containing nisin. The native nisin ABC-ring system was obtained via chymotrypsin digestion of full-length nisin, and was subsequently functionalized at the C-terminal carboxylate with two different amino alkyne moieties. Next, nisin hybrids were successfully prepared using Cu(I)-catalyzed azide alkyne cycloaddition 'click' chemistry by chemo-selective ligation of the ABC-alkyne with the N-terminal azido functionalized dicarba-DE ring mimic. The newly synthesized compounds were active as potent lipid II binders and retained antimicrobial activity in a growth inhibition assay. However, pore formation was not observed, possibly either due to the different character of the 'staples' as compared to the parent sulfides, or due to the triazole moiety as a sub-optimal amide bond isostere., (Copyright © 2014 Elsevier Ltd. All rights reserved.)

Herein, we describe the synthesis, structural characterization, and synthetic use as an advanced intermediate of a cross-stapled alkene-bridged hexapeptide to mimic the DE-ring of the lantibiotic nisin. The linear precursor was cyclized by ring-closing metathesis to give the correctly folded bicyclic hexapeptide in a single step, and the four individual diastereoisomers were isolated, structurally assigned and characterized by HPLC, NMR and MS, respectively. The bicyclic hexapeptide was used as a versatile advanced synthon and was modified at its C- and N-terminus, among others, with an azide moiety to access a building block suitable for Cu(I)-catalyzed alkyne-azide cycloaddition-based ligation reactions.

Herein, we describe a scalable purification of the lantibiotic nisin via an extraction/precipitation approach using a biphasic system, which can be carried out up to 40-80 gram scale. This approach results in an at least tenfold enrichment of commercially available preparations of nisin, which usually contain only 2.5% of the desired peptide, to allow further purification by preparative HPLC. As a follow-up study, the enriched nisin sample was digested either by trypsin or chymotrypsin, or treated by CNBr, and these reactions were monitored by LC-MS to identify and characterize the obtained fragments. Two previously unknown cleavage sites have been identified: Asn20-Met21 and Met21-Lys22 for trypsin and chymotrypsin, respectively. Furthermore, a novel and convenient enzymatic approach to isolate the native nisin C-ring [nisin fragment (13-20)] was uncovered. Finally, by means of preparative HPLC, nisin fragments (1-12), (1-20), (22-34), and (22-31) could be isolated and will be used in a semi-synthesis approach to elucidate the role of each fragment in the mode of action of nisin as an antimicrobial peptide., (Copyright © 2013 European Peptide Society and John Wiley & Sons, Ltd.)

The lipophilic amino acid, (S)-2-aminoundecanoic acid, was synthesized and incorporated at a number of specific positions within the peptide sequence of anoplin. These lipophilic anoplin analogs showed to be more active against Escherichia coli and Staphylococcus aureus compared to native anoplin, while the EC50-value of hemolysis was at least one order of magnitude lower than the MIC values. This was in sharp contrast to the N-acylated anoplin derivative, where a gain in activity also led to a complete loss of selectivity. Thus, the incorporation of a lipophilic amino acid residue into anoplin enhanced the antimicrobial activity, while selectivity towards microbial membranes was retained., (Copyright © 2013 Elsevier Ltd. All rights reserved.)