MISCELLANEOUS NITROGEN AND SULFUR COMPOUNDS 283 



H S SH 



RNCS + R'OH > N— C— or' < > RN=C-0R' 



R 



Usually R'OH is actually part of the RNCS molecule so that the reaction is intramolecular, 

 and a heterocyclic ring is produced, or polymers may form. These complex reactions 

 need much more clarification at the single-step, enzymatic level. They have been indi- 

 cated here in order to show that four different types of compound may all arise as sec- 

 ondary products from a single glucoside precursor. Specific examples will be given 

 below. 



The mustard oil glucosides are widespread among the Cruciferae but are occasion- 

 ally found in other plant families as well. About thirty different ones are presently known, 

 and most plants which have them have more than one. There is good reason to believe 

 that they function to protect plants against parasites. It is, of course, the aglycones 

 formed enzymatically upon cellular disintegration which are the active compounds in this 

 respect. Virtanen (26) describes the mustard oils as the most active antibiotics found 

 in higher plants. Goitrin (see below for structure) which occurs in Brassica spp. is 

 also of economic interest since it acts as a goitrogenic or antithyroid compound toward 

 animals (27). 



In Table 2 some mustard oil glucosides are listed together with their enzymatic hy- 

 drolysis products and selected species in which they occur. It will be noted that in differ- 

 ent species the same glucoside (e.g. sinigrin) may form different hydrolysis products. 

 It should also be mentioned that although the hydrolysis products are usually called "mus- 

 tard oils" and thought of as being volatile and odoriferous, some are non-volatile solids. 

 The complex glycoside glucobrassicin is mentioned below under "Indole Derivatives". 



Only a few of the original glucosides have been isolated and purified. Most work 

 has been done with the hydrolysis products, which are more easily obtained. In order to 

 obtain the unchanged glucosides from plants it is necessary to inactivate the myrosinase 

 by heating to 100° C. Glucosides may be extracted with boiling water, ethanol, methanol 

 etc. If lipids are also present, as in some seeds, they should first be removed with fat 

 solvents. Crude glucoside solutions are concentrated, chromatographed on alumina or 

 anion exchange resin and crystallized from alcohol-water. Addition of silver nitrate to 

 the glycosides cleaves off the glucose with precipitation of a silver salt: 



^NOSOsAg 

 R-CC' 



SAg 



This can be converted to the corresponding isdthiocyanate by shaking with aqueous sodium 

 thiosulfate. Volatile mustard oils are prepared by macerating plant parts in water for 

 several hours to permit enzymatic hydrolysis to occur, then steam distilling or extract- 

 ing with ether to obtain the volatile oil. Further purification can be achieved by fractional 

 distillation or column chromatography. Non-volatile hydrolysis products such as goitrin 

 have been purified by extraction with water and ether (28). 



Several simple spot reactions have been developed for characterization of the mus- 

 tard oils and their glucosides. For the volatile ones their sharp odor and biting taste 

 are distinctive enough to indicate their presence. Paper chromatography has been of 

 great assistance in the characterization of these compounds. The glucosides themselves 



