Abstract:
The world is currently in an era of antimicrobial resistance (AMR), where pathogenic microbes are developing resistance to our treatments. Lipopeptides have been touted as a potential solution of the rise of AMR due to their non-specific mechanisms of action. This is exemplified by the clinically-approved Cubicin and polymyxin B, where they act as the last-line defence agents against “superbugs” that are resistant against multiple drugs. However, their toxic side effects have limited their use. Hence, there is a need for the synthesis of new generation lipopeptides free of toxic properties. The lipid moiety of lipopeptides has been the main obstacle in accessing this class of compounds, due to the challenging synthetic routes towards lipoamino acid building blocks needed for the peptide sequence. This step can be replaced with a convergent approach, where the lipid moiety is conjugated onto a peptide. As a replacement, our group developed a one-step method for introducing lipids onto peptides: Cysteine Lipidation on a Peptide or Amino acid (CLipPA). CLipPA is a one-step reaction between a vinyl ester bearing a lipid and a thiol on a peptide or amino acid. Through the CLipPA reaction, we synthesized a library of novel analogues based on the naturally occurring AMPs battacin, teixobactin and iturin A. The hydrophobic regions of these AMPs are replaced with a thiol containing amino acid, and an array of vinyl esters are introduced to generate a series of compounds based on battacin, teixobactin and iturin A. 12 novel analogues of battacin were synthesized, and the most active compound was able to inhibit and kill Gram-positive and Gram-negative bacteria at low micromolar concentrations (MBC = 8 – 32 μM). Six novel analogues of teixobactin were synthesized. The hydrophobic region of residues were replaced with lipids introduced by CLipPA. The analogues were unable to replicate the antibacterial properties of teixobactin. However, it further showcased the versatility CLipPA by applying CLipPA onto a structurally different peptide precursor. Six novel analogues of antifungal peptide iturin A were synthesised. The lipid tail stems directly from a residue embedded within the macrocycle of iturin A. By exploiting this structural feature, we synthesized the cyclic scaffold using native chemical ligation (NCL), whereupon the regenerated thiol vinyl esters were conjugated with the CLipPA reaction and constructed a series of cyclic lipopeptides. The work in this thesis has further showcased the versatility of the CLipPA reaction where it was applied onto three distinct peptide scaffolds. The chemoselective reaction could be applied onto other mimetics of lipopeptides in the screening for new effective antimicrobial peptides. Lastly, the biological results of battacin CLipPA analogues reveal that this synthetic linkage can imitate pore-forming abilities commonly exhibited by antimicrobial peptides.