Observation of a stable carbene at the active site of a thiamin enzyme.

Carbenes are highly reactive chemical compounds that are exploited as ligands in organometallic chemistry and are powerful organic catalysts. They were postulated to occur as transient intermediates in enzymes, yet their existence in a biological system could never be demonstrated directly. We present spectroscopic and structural data of a thiamin enzyme in a noncovalent complex with substrate, which implicate accumulation of a stable carbene as a major resonance contributor to deprotonated thiamin.