Synthesis and structural studies of macrocyclic antimicrobial peptides

Abstract

Antimicrobial resistance (AMR) poses a major global health and economic threat, with projections estimating 50 million deaths annually by 2050 if unaddressed. Unfortunately, most clinically approved antimicrobials have been on the market for decades and therefore have at least one, if not multiple, known resistance mechanisms. Antimicrobial peptides (AMPs), naturally occurring molecules with diverse structures and modifiable backbones, offer a promising source of new antibiotics. This research focuses on two cyclic lipopeptides (CLiPs), globomycin and brevicidine, both active against Gram-negative bacteria.
Previous synthetic methods for globomycin relied on solution-phase or fragment-based strategies, this study explored an on-resin approach using side-chain immobilisation to streamline synthesis. Despite extensive condition screening, a fully on-resin method was ultimately unfeasible. However, the project led to the successful development of a stereoselective strategy for synthesising complex disubstituted lipids via cross-metathesis, providing a flexible synthetic template for producing diverse lipid building blocks useful throughout natural product synthesis.

To improve globomycin’s stability, a library of twelve analogues was created by replacing its hydrolysis-prone ester linkage with amide and olefin bonds. These analogues were synthesised using solid-phase peptide synthesis (SPPS), but none retained antimicrobial activity or inhibited the target enzyme LspA, highlighting the challenges in AMP optimization.

Brevicidine and laterocidine, discovered through genome mining of biosynthetic gene clusters (BGCs), show potent activity against Gram-negative pathogens, including ESKAPE species. Both peptides demonstrate strong proteolytic stability and low toxicity. This ushered in an expansive investigation into the structural optimisation of these peptides to seek improvements to their antimicrobial activity, stability and toxicity profiles. This was a collaborative project carried out alongside the Martin group (Leiden University), who focused on all laterocidine analogues. N-terminal lipophilicity was assessed to ascertain optimum lipid length and a full alanine scan conducted to identify key residues.

Date of Award	Jul 2025
Original language	English
Awarding Institution	Queen's University Belfast
Sponsors	Engineering and Physical Sciences Research Council
Supervisor	Stephen Cochrane (Supervisor), Peter Knipe (Supervisor) & Ross Ballantine (Assistant Supervisor)