Cysteine-carbazole reaction

It was found that carbazole and phloroglucinol are unsuitable for determining the heptose content of sugar mixtures because of interference from other sugars similar interference is encountered with resorcinol. With orcinol, cysteine, and diphenylamine, however, specific reactions can be obtained at selected wavelengths. Ketoheptoses (and keto sugars in general) react under milder acid conditions than the aldoses, and this property can be used for identification purposes. In the orcinol test, for... 

Chemical Methods. Determinations of galacturonic acid of pectin usually includes both the free and esterified forms, since strongly acidic media are employed in the colorimetric methods. Procedures which continue to be used widely are modifications of those described early by Dische. One is based upon reaction with cysteine (3) and the other with carbazole (4). One modification of the carbazole method, which gave a doubling in sensitivity, along with an increased stability in color and greater reproducibility, was reported in 1962 (5). Inclusion of borate into the assay medium was responsible for the enhancement of the method. 

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