Tautomerization ensures quality clicks

Chemists looking to develop new techniques in bioorthogonal chemistry can encounter a kind of paradox. Stable, biocompatible molecules are required for use in living tissues. But the reactants must also work swiftly and selectively when called upon—for example, to release a cancer drug in a tumor or attach an imaging label to a target protein.

Some previous studies have looked to molecules that can be triggered by light or oxidation to control their reactivity, but while these strategies work well in the culture dish, they would be tricky to implement in more complex settings, such as inside a human body.

Now, Qing Lin and his coworkers at the University at Buffalo describe a new strategy to strike that elusive balance between stability and reactivity using a mainstay of organic chemistry: the type of interconvertible isomers known as tautomers (J. Am. Chem. Soc. 2023, DOI: 10.1021/jacs.2c12325).

The researchers took nitrile imines, which eagerly undergo click cycloaddition with strained alkynes, and devised a stable tautomeric form called a hydrazonyl sultone, in which the imine carbon is tucked into a ring. When the hydrazonyl sultone meets an eligible alkyne, it will open up into the nitrile imine and do the click reaction.

“The substance will be stable in the absence of its reaction partner but becomes active when it sees its reaction partner,” Lin says. “That’s almost an ideal situation.”

The researchers systematically examined how the sultone ring size and modifications on the adjoining aromatic ring affected the molecules’ stability and reactivity with an alkyne partner. They found that a six-membered sultone ring with two methyl groups adjacent to the sulfur was particularly good for stability.

After arriving at a molecule that hit the stable-yet-reactive sweet spot, the researchers tried attaching it to proteins. They found that the more tyrosines were near the reaction site, the faster the reaction went. Lin hypothesizes that the tyrosines create a sort of binding pocket that encourages the molecule to transform into the reactive form, but he says the mechanistic impacts of the protein microenvironment need further investigation.

The team also successfully used the reaction to attach fluorescent labels to membrane proteins in live cells.

The way Lin and his team harnessed the tautomer transformation is “super clever,” says Joseph M. Fox, a bioorthogonal chemistry researcher at the University of Delaware who was not involved in the work. “I’m a big proponent of physical organic chemistry intersecting with applications in biology.”

Neal Devaraj, a professor at the University of California San Diego who also studies bioorthogonal chemistry, says he envisions this reaction being useful for selective labeling because of the way it responds to proteins’ microenvironments. “I’ll be really curious to see how they can exploit that,” he says.

Lin says his team is continuing to optimize the molecular motif to balance stability and reactivity and is moving toward trying the reaction in mice soon. He hopes to eventually develop it for targeted immuno-positron emission tomography imaging.

Bioorthogonal chemistry earned a Nobel nod last year, but the field is not going to rest on its laurels, Lin says. “We need to keep pushing this field.”