280 



MISCELLANEOUS NITROGEN AND SULFUR COMPOUNDS 



Upon distillation the sulfoxide, allicin, forms diallyl-disuLfide, diallyltrisulfide and 

 allyl propyl disulfide which are constituents of commercial garlic oil. Some such series 

 of reactions probably accounts for the presence of disulfides in other similar oils. Even 

 carbon disulfide has been reported in cabbage (13). 



AUiin was the first natural sulfoxide to be discovered, but a few others have since 

 been found. Some sulfoxide amino acids are included in Chapter 10. Other natural sul- 

 foxides are also isothiocyanates and will be mentioned with this group of compounds. 

 Further oxidation of a sulfoxide yields a sulfone (RSOjR'). Only a few natural sulfones 

 are known, and they are regarded as secondary products derived from sulfoxides. A 

 sulfonic acid, sulfoacetic acid, has been ioundin Erytlirina spp. (14), and another sul- 

 fonic acid is present as the sulfolipid of chloroplasts (15). 



The sulfoxides are of interest because of their antibiotic action. Allicin is bacteri- 

 ocidal and also inhibits several enzymes in vitro (16). In onion S-methyl-and S-n-pro- 

 pylcysteine -sulfoxides occur. When the onion is crushed, these are enzymatically con- 

 verted to thiosulfinates which have an even stronger anti-microbial action (17): 



NHo CHg CH3 



CH3SCH2CHCOOH > S S 







This reaction is clearly analogous to the formation of allicin shown previously. Sulfur 

 compounds of food plants have recently been reviewed by Virtanen (18). 



The thetins are metabolically the most interesting of the compounds discussed in 

 this section since they are able to act as donors of methyl groups for several important 

 methylation reactions (19). They are suKonium salts analogous to the quaternary nitro- 

 gen bases. Like the quaternary nitrogen compounds they may be precipitated as Rein- 

 eckates, picrates, etc. Dimethyl-/3-propiothetin: 



+ 

 (CH3)2SCH2CH2COOH 



was the first sulfonium compound to be found in plants. It is rather common in algae, 

 but has not been reported in higher plants. Similar sulfonium compounds may, however, 

 be found in Eqiusetinn spp. and some ferns (20). On treatment with cold alkali or with 

 an enzyme present in some marine algae dimethyl-/3-propiothetin decomposes as follows: 



(CH3)2SCH2CH2COOH - (CH3)2S + CH2 = CHCOOH + H+ 



Where dimethyl sulfide is found as a natural product, it probably results from a similar 

 reaction. Methylmethioninesulfonium salts have been detected in a variety of plants. 

 They release dimethyl sulfide on treatment with boiling alkali. 



The simple thiols and sulfides are generally isolated by taking advantage of the 

 insoluble salts or complexes which they form with certain metal ions, primarily mer- 

 curic. Insoluble mercaptides are formed by reacting thiols with mercuric chloride or 

 cyanide. Sulfides form insoluble corrdination complexes with mercuric chloride but not 

 with mercuric cyanide. This difference in reactivity may be used to separate mercaptans 

 from sulfides. The original compounds are regenerated by treating the insoluble precipi- 

 tates with acid. As noted before, disulfides are split by mercuric salts; so that they may 

 not be isolated in their original form by this method. However, it must be recalled that 



