Slicing new sulfur chemical from onion.

Slicing new sulfur chemical from onion

Extracts of blended onion contain a brew of over 100 sulfur-containing compounds, which chemists have been identifying for about the same number of years.

In the May 23 JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, Eric Block and Thomas Bayer of the State University of New York at Albany report their discovery of a previously unknown onion chemical that should help flesh out more of the intricate onion chemistry puzzle. It also shows a moderate test-tube ability to prevent the biochemical cascade that leads to asthma and inflammatory reactions.

In Block's assessment, "Onions contain the most bizarre and exotic sulfur compounds that have ever been made synthetically or found in nature." In the late 1970s, he identified (Z)-propanethial S-oxide, the onion chemical that makes people tear up in the kitchen and his lab.

The compound forms when the vegetable gets damaged or sliced, thus enabling the enzyme allinase to convert precursors into the tear-jerking chemical. It then undergoes a series of spontaneous reactions to automatically produce the rest of the onion's sulfurous chemical brew.

Until now, propanethial S-oxide was the only known naturally occurring chemical containing an atomic arrangement called a sulfine group. The four-atom group consists of a sulfur atom bonded on one side to an oxygen atom and on the other side to a carbon atom, in turn double-bonded to another carbon. The newly identified compound, called (Z,Z)-d,1-2,3-dimethyl-1,4-butanedithial S,S'-dioxide, has a double dose of this unusual structure, Block notes. Knowing this compound's structure should help the researchers figure out which of the other onion chemicals emerge from it.