On the Lachrymatory Factor in Onion (Allium cepa) Vapours and Its Precursor.

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Abstract
The isolation of the precursor of the lachrymatory factor in onion is described in detail. The compound was characterized as (+) -S-(prop-l-enyl)-L-cysteine sulphoxide. An enzyme preparation of onion splits it into the lachrymatory factor, pyruvic acid, and ammonia. The lachrymatory factor is very unstable. In mass-spectral studies of the enzymic cleavage of the lachrymatory precursor no higher mass number than 90 could be detected. These studies were in accordance with the idea that the lachrymatory factor is propenylsulphenic acid. It is spontaneously degradated to propional-dehyde from which some 2-methyl-2-pentenal is formed. In mild alkaline solution the precursor of the lachrymatory factor is cyclized to cycloalliin.

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