The aglucone of sinigrin

[摘要] The mechanism of the ascorbate-activated cleavage of sinigrin to mustard oil has been postulated as enzymatic hydrolysis of the thioglucoside to the aglucone, which undergoes spontaneous Lossen rearrangement to allyl isothiocyanate. Potassium silver sinigrate (I) served as a convenient source of the aglucone (II) for comparison. When the potassium salt was decomposed with iodide at pH 2, 97% of the product was allyl cyanide; at pH 5, 97% was allyl isothiocyanate; and at pH 3.53, the two compounds were formed in equal amounts. The ratio of nitrile to isothiocyanate was proportional to hydrogen ion concentration. Enzymatic hydrolysis of sinigrin at pH 3.2-5, with or without ascorbate, gave essentially the same product distribution. Treatment of potassium silver sinigrate with hydrogen cyanide at pH 6.6 allowed spectroscopic observation of the aglucone. It (II) has an ultraviolet absorption maximum at 245 mu, E11,000, and a half-life of 37 seconds at 20° (estimated 1 minutes at 0°). These results agree with data for and the resulting aglucone either undergoes Lossen rearrangement or is captured by another enzyme (;;thiocyanase;;) and converted to allyl thiocyanate. Isothiocyanate once formed is not isomerized. That the thiocyanate is derived from the aglucone was demonstrated by reaction of potassium silver sinigrate with iodide and ground Thlaspi seed. The aglucone thus produced was converted efficiently to thiccyanate. Allyl isothiocyanate and allyl thiocyanate reached an equilibrium in the undiluted liquid, with 5.1% allyl thiocyanate at 24° and 8.2% at 100°. Equilibrium is approached at 24° with a half-life of 17-19 days. In dilute aqueous and methanolic solutions, which decompose rapidly, the apparent equilibrium is 35-40% allyl thiocyanate.

[发布机构] Rice University