Lipidated cyclopropenes via a stable 3-N spirocyclopropene scaffold

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.

Modular activatable bioorthogonal reagents

The bioorthogonal reaction toolbox contains approximately two-dozen unique chemistries that permit selective tagging and probing of biomolecules. Over the past two decades, significant effort has been devoted to optimizing and discovering bioorthogonal reagents that are faster, fluorogenic, and orthogonal to the already existing bioorthogonal repertoire. Conversely, efforts to explore bioorthogonal reagents whose reactivity can be controlled in space and/or time are limited. The "activatable" bioorthogonal reagents that do exist are often unimodal, meaning that their reagent's activation method cannot be easily modified to enable activation with red-shifted wavelengths, enzymes, or metabolic-byproducts and ions like H₂O₂ or Fe³⁺. Here, we summarize the available activatable bioorthogonal reagents with a focus on our recent addition: modular caged cyclopropenes. We designed caged cyclopropenes to be unreactive to their bioorthogonal partner until they are activated through the removal of the cage by light, an enzyme, or another reaction partner. To accomplish this, their structure includes a nitrogen atom at the cyclopropene C3 position that is decorated with the desired caging group through a carbamate linkage. This 3-N cyclopropene system can allow control of cyclopropene reactivity using a multitude of already available photo- and enzyme-caging groups. Additionally, this cyclopropene scaffold can enable metabolic-byproduct or ion activation of bioorthogonal reactions.