In July 2017’s journal ACS Chemical Biology, M Golczak and J Silvaroli show what happens when you cut an onion.

chopping-onions.jpg  caa6c293e4014a73ac16e5ff70ab50d3.jpg

A so-called Lachrymatory Factor (LF) is formed by the release of two chemicals that are released by the cutting of cell walls. One is a sulfenic acid precursor. The other is an enzyme called alliinase which is normally contained within the cells’ vacuoles. Upon release the two react to form something like tear gas.

plant vacuole.GIF

garlic, an alliinase enzyme acts on the chemical alliin converting it into allicin. The process involves two stages: elimination of 2-propenesulfenic acid from the amino acid unit (with α-aminoacrylic acid as a byproduct), and then condensation of two of the sulfenic acid molecules.

Reaction scheme for the conversion: cysteine → alliin → allicin

There are a range of similar enzymes that can react with the cysteine-derived sulfoxides present in different species. In onions, an isomer of alliin, isoalliin, is converted to 1-propenesulfenic acid. A separate enzyme, the lachrymatory factor synthase or LFS, then converts this chemical to a potent LF.

But none of this mattered to Roy Orbison