Cystine ninhydrin reaction

Water-soluble a-DNP-Arg, a-DNP-His, s-DNP-Lys, bis-DNP-His, O-DNP-Tyr, DNP-cysteic acid (CySOjH), and DNP-cystine (Cys)2 have been identified on silica gel plates in the n-propanol-34% ammonia (7 3 v/v) system. Although separation of DNP-Arg and s-DNP-Lys is incomplete Rf values of 0.43 and 0.44, respectively), both of them can be detected because of the color difference produced in the ninhydrin reaction. 

Analytical procedures involving reduction and determination of mercaptan are not accurate determinations of cystine in permanent-waved hair or in hair treated with mercaptan because mixed disulfide is reduced to mercaptan during analysis adsorbed mercaptan can also interfere in the determination. Procedures that do not involve reduction of hair such as ninhydrin detection (alpha-amino group) or dinitrofluorobenzene (DNFB) reaction followed by chromatographic separation [1, 58] discriminate between mercaptans and, therefore, should be better analytical procedures for detecting the different types of mercaptans and disulfides actually present in permanent-waved hair. 

In the case of amino acid analysis, the quantification of cysteine can be difficult because it is oxidized to cystine during acid hydrolysis. To circumvent this problem, cysteine can be oxidized to cysteic acid with performic acid prior to analysis. Alternatively, cysteine can be converted to the pyridyl ethyl derivative and subsequently detected by postcolumn reaction with ninhydrin. Still another method involves the production of carboxymethyl cysteine following alkylation with iodoacetic acid. All of these cysteine derivatives can be separated by either reversed-phase precolumn or ion-exchange postcoT umn methods. 

Bovine vasopressin does not react with ninhydrin. It gives a reaction for tyrosine (a-nitroso-j3-naphthol), cystine (iodoplatinate), and arginine (Sakaguchi) (11),... 

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