Click chemistry

Click chemistry is a class of chemical reactions defined by high yield, broad scope, oxygen and water tolerance, and a thermodynamic drive that lets them proceed cleanly under mild conditions. The 2022 Nobel Prize in Chemistry was awarded for click chemistry and bioorthogonal chemistry.

Background

The phrase was introduced by K. Barry Sharpless in 2001 to describe reactions that connect (click) two molecular fragments together with the reliability of a seatbelt buckle. Defining criteria: simple to perform, high product yield, minimal side products, no need for protecting groups, broadly applicable.

Two specific click reactions matter for haircare:

• Thiol-ene — a thiol (R-SH) reacts with an alkene (C=C) under mild conditions to form a new C–S bond. Wolff (1900) discovered the mechanism; the modern click-chemistry framing is Sharpless's.
• Thiol-yne — same general idea but with an alkyne (C≡C). The triple bond can react twice, allowing one molecule to form two new C–S bonds.

In hair, free thiol groups appear on cysteine residues when disulfide bonds break (S–S → 2 S–H). Click-chemistry molecules carrying alkene or alkyne functional groups react with those free thiols and form new covalent crosslinks at the damage sites. The reaction class is well-characterized in peer-reviewed literature.

The 2022 Nobel laureates: Carolyn R. Bertozzi (Stanford), Morten Meldal (University of Copenhagen), and K. Barry Sharpless (Scripps Research). Sharpless's second Nobel in Chemistry — only five people have ever won the Nobel twice, and he is the second to do so in the same field.