MISCELLANEOUS NITROGEN AND SULFUR COMPOUNDS 



281 



Simple disulfides are probably artifacts rather than true natural products; and their pre- 

 cursors (i. e. amino acid sulfoxides) may be isolated by the methods used for other amino 

 acids. The sulfonium compounds are distinguished from other sulfur compounds by their 

 positive charge, and may therefore be separated on cation exchange resins or precipitated 

 by complex anions such as Reineckates, chloroplatinates, phosphotungstates, etc. The 

 original compound is then regenerated by treatment of the precipitate with acid. No pro- 

 cedures can be discussed in general terms for isolating the more unusual sulfur compounds 

 since each one represents a special case. 



Characterization of the simple sulfur compounds has relied heavily on the reactions 

 with mercuric salts which have been described above. Basic amines may also be precipi- 

 tated as mercury complexes. However, by distilling from an acidic mixture and passing 

 volatile compounds into a solution of mercuric salt amines will be left behind and cause 

 no interference. Under these conditions the appearance of a precipitate may be taken as 

 evidence for simple sulfur compounds. It should also be noted that in many cases odor 

 alone may be sufficient evidence for distinguishing between amines and volatile sulfur 

 compounds. 



For distinguishing among the different classes of sulfur compounds specific modifi- 

 cations of the mercury reaction have been developed. Other special reagents are also 

 available. The best known of these is alkaline sodium nitroprusside solution which gives 

 a purple color with sulfhydryl compounds. If disulfides are present, preliminary treat- 

 ment with potassium cyanide splits the S-S bond so that nitroprusside is able to react. 

 Amines also react with nitroprusside in the presence of carbonyl compounds such as ace- 

 tone or acetaldehyde. The presence of methyl sulfonium compounds is indicated by the 

 evolution of dimethyl sulfide on treating with alkali. Some sulfonium compounds require 

 boiling with sodium hydroxide solution; others release dimethyl sulfide in the cold. 



Because of their volatility the low molecular weight sulfur compounds are unsuited 

 to paper chromatography. Gas chromatography has been useful in characterizing them 

 (13). The non-volatile sulfur compounds may be separated and identified by paper chro- 

 matography. Valuable methods for this purpose have been described by Toennies and 

 Kolb (21). A platinum chloride-potassium iodide reagent was found most generally valu- 

 able for detecting sulfur compounds on chromatograms although it does not react well 

 with sulfoxides. Nitroprusside was used for sulfhydryl compounds and nitroprusside fol- 

 lowed by sodium cyanide for disulfides. 



A few suggestions have been made regarding the biosynthetic pathways of simple 

 sulfur compounds, but not enough is known to attempt construction of a metabolic map. 

 It may be assumed that sulfur amino acids are precursors, but the nature and number of 

 enzymatic steps leading from them can only be guessed at. A general review of sulfur 

 metabolism can be found in the book of Young and Maw listed under "General References." 

 However, this gives very slight treatment to higher plants. 



MUSTARD OIL GLYCOSIDES 



The mustard oils have been known for many years and are economically important 

 as the flavor constituents of such condiments as mustard, horseradish, and water cress. 

 For the most part they are colorless liquids with a sharp, irritating odor and the ability 

 to raise blisters on the skin. Before 1900 it was understood that the mustard oils are 

 actually secondary products arising from breakdown of glucosides when cellular struc- 

 ture is disrupted. In addition to the general references, reviews of these compounds may 

 be found in articles by Kjaer (22) and Zinner (23). It seems probable that a single gen- 

 eral structure can be written for all glucosides of this group. Upon hydrolys.is, however, 

 the aglycones undergo rearrangement. In some cases the rearrangements are so exten- 

 sive that the final product bears no apparent resemblance to the aglycone as it exists in 

 the glucoside. 



