Employing Sterculic Acid – a naturally occurring 1,2-cyclopropene fatty acid – for live-cell imaging of lipids by inverse electron-demand Diels-Alder reaction

Bioorthogonal chemistry has allowed the study of biomolecules in living systems with minimal structural perturbations to the molecules under investigation. In the field of lipid research, this has allowed for the study of lipid uptake and processing, whilst minimising artefacts on their biology resulting from pendant detectable 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 here show that this lipid can be readily taken up by dendritic cells without toxic side-effects, and that it can subsequently be visualised in live cells using an inverse electron-demand Diels-Alder (IEDDA) reaction with quenched tetrazine fluorophores. Furthermore, 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.