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