Live-Cell Imaging of Sterculic Acid-a Naturally Occurring 1,2-Cyclopropene Fatty Acid-by Bioorthogonal Reaction with Turn-On Tetrazine-Fluorophore Conjugates.

In the field of lipid research, bioorthogonal chemistry has made the study of lipid uptake and processing in living systems possible, whilst minimising biological properties arising from detectable pendant groups. To allow the study of unsaturated free fatty acids in live cells, we here report the use of sterculic acid, a 1,2-cyclopropene-containing oleic acid analogue, as a bioorthogonal probe. We show that this lipid can be readily taken up by dendritic cells without toxic side effects, and that it can subsequently be visualised using an inverse electron-demand Diels-Alder reaction with quenched tetrazine-fluorophore conjugates. In addition, the lipid can be used to identify changes in protein oleoylation after immune cell activation. Finally, this reaction can be integrated into a multiplexed bioorthogonal reaction workflow by combining it with two sequential copper-catalysed Huisgen ligation reactions. This allows for the study of multiple biomolecules in the cell simultaneously by multimodal confocal imaging.

Thrombin-Derived Peptides Potentiate the Activity of Gram-Positive-Specific Antibiotics against Gram-Negative Bacteria.

The continued rise of antibiotic resistance threatens to undermine the utility of the world's current antibiotic arsenal. This problem is particularly troubling when it comes to Gram-negative pathogens for which there are inherently fewer antibiotics available. To address this challenge, recent attention has been focused on finding compounds capable of disrupting the Gram-negative outer membrane as a means of potentiating otherwise Gram-positive-specific antibiotics. In this regard, agents capable of binding to the lipopolysaccharide (LPS) present in the Gram-negative outer membrane are of particular interest as synergists. Recently, thrombin-derived C-terminal peptides (TCPs) were reported to exhibit unique LPS-binding properties. We here describe investigations establishing the capacity of TCPs to act as synergists with the antibiotics erythromycin, rifampicin, novobiocin, and vancomycin against multiple Gram-negative strains including polymyxin-resistant clinical isolates. We further assessed the structural features most important for the observed synergy and characterized the outer membrane permeabilizing activity of the most potent synergists. Our investigations highlight the potential for such peptides in expanding the therapeutic range of antibiotics typically only used to treat Gram-positive infections.

The contribution of achiral residues in the laspartomycin family of calcium-dependent lipopeptide antibiotics.

The growing threat of antibacterial resistance is a global concern. The so-called calcium-dependent lipopeptide antibiotics (CDAs) have emerged as a promising source of new antibiotic agents that are rich in structural and mechanistic diversity. Over forty unique CDAs have been identified to date and share a number of common features. Recent efforts in our group have provided new mechanistic and structural insights into the laspartomycin family of CDAs. We here describe investigations aimed at probing the role of the three glycine residues found in the laspartomycin peptide macrocycle. In doing so laspartomycin analogues containing the achiral 2-aminoisobutyric acid (AIB) as well as l- or d-alanine in place of glycine were prepared and their antibacterial activities evaluated.

Mechanistic insights into the C55-P targeting lipopeptide antibiotics revealed by structure-activity studies and high-resolution crystal structures.

The continued rise of antibiotic resistance is a global concern that threatens to undermine many aspects of modern medical practice. Key to addressing this threat is the discovery and development of new antibiotics that operate by unexploited modes of action. The so-called calcium-dependent lipopeptide antibiotics (CDAs) are an important emerging class of natural products that provides a source of new antibiotic agents rich in structural and mechanistic diversity. Notable in this regard is the subset of CDAs comprising the laspartomycins and amphomycins/friulimicins that specifically target the bacterial cell wall precursor undecaprenyl phosphate (C55-P). In this study we describe the design and synthesis of new C55-P-targeting CDAs with structural features drawn from both the laspartomycin and amphomycin/friulimicin classes. Assessment of these lipopeptides revealed previously unknown and surprisingly subtle structural features that are required for antibacterial activity. High-resolution crystal structures further indicate that the amphomycin/friulimicin-like lipopeptides adopt a unique crystal packing that governs their interaction with C55-P and provides an explanation for their antibacterial effect. In addition, live-cell microscopy studies provide further insights into the biological activity of the C55-P targeting CDAs highlighting their unique mechanism of action relative to the clinically used CDA daptomycin.