278 MISCELLANEOUS NITROGEN AND SULFUR COMPOUNDS 



THIOLS AND SULFIDES 



The simple alkyl thiols (mercaptans) and sulfides have much in common with the 

 amines discussed in the previous section. They are both volatile, with an offensive odor; 

 and biogenetically they are probably both derived from amino acids. The basic struc- 

 tures of the sulfur compounds to be discussed in this section are as follows: 



Examples of all of these types occur in higher plants, and many more probably remain 

 to be discovered. Some of the best known ones appear to be artifacts which are formed 

 during the isolation process. The sulfur amino acids are treated in Chapter 10. The 

 simple mercaptans are readily soluble in aqueous alkali and form insoluble mercaptides 

 with many heavy metal cations. The name mercaptan was originally given to them because 

 of the readiness with which they form mercury salts. Mercaptans are readily oxidized 

 in the air to disulfides, so that it may be advisable to maintain a nitrogen atmosphere 

 while working with them. Methyl mercaptan (methanethiol) is found in radish (Raphanus 

 spp.) roots; n-propyl mercaptan in onions (Allium cepa). 2, 2'-dithioisobutyric acid and 

 the corresponding disulfide are found in asparagus: 



HSCHj^ 



CHCOOH 

 HSCH2 



An unusual thiol of the following structure has been reported in cabbage (Brassica 

 oleracea) (10): 



HS 

 HS 



P 



other thiol compounds are known to have important metabolic functions (e. g. coenzyme 

 A, thioctic acid), but no metabolic role has been assigned to the simple mercaptans 

 unless they serve an ecological function in repelling some herbivores or parasites. 



The dialkyl sulfides are less volatile than the mercaptans and have a less unpleas- 

 ant odor. With mercuric salts they form coordination complexes rather than true salts. 

 Some essential oils have been reported to contain dimethyl sulfide, and divinyl sulfide is 

 the major constituent of ramsons oil (Allium ursinum). Pineapple juice (Ananas comosus) 

 has the methyl ester of 2-methylthiopropionoic acid. Garlic oil which was formerly 

 believed to contain diallyl sulfide is now known to have only di- and polysuLfides. Di- 

 methyl sulfide probably occurs more frequently than other sulfides because it is a de- 

 composition product of the thetins (q. v. ). Cyclic sulfides have been found in plants of the 

 family Compositae . Obviously related terthienyl, and bithienyl compounds are found in 



