Cysteine specific color reaction

Figure 8 outlines possible routes leading to cysteine specific products via 1-deoxypentosone. Under the chosen reaction conditions, (180 °C H-O) norfuraneol is further transformed via 4-hydroxy-5-metnyl-3-(2H)-thiophenone and 2-methyl-3-(2H)-thiophenone, into 2-methyl-3-mercapto-thiophene and 2-methylthiophene, respectively. Norfuraneol is a methylene active compound, which undergoes aldolcondensation with carbonyls forming colored nonvolatile products. 2-Mercaptopropionic acid may be formed from the mercaptoketone by oxidative cleavage. By this route 2-methyl-3-thiolanone and 2-methyl-4,5-dihydro-thiophene may result. 

Reaction with thiols. The reagent reacts specifically with the thiol group of an amino acid or protein to give a derivative (2) which is colored or develops color with ninhydrin. Thus the sulfhydryl group of cysteine adds to the activated double bond to give a yellow neutral compound (2). This compound, which gives a purple color with ninhydrin, rearranges even in neutral solution to the carboxylic acid (3).1... 

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