ON THE MECHANISM OF THE REACTION OF NINHYDRIN WITH a-AMINO ACIDS

Our purpose is to show by means of a quantitative method for determination of hydrindantin derivatives that previous concepts of the mechanism of the reaction of ninhydrin with amino acids are inadequate or erroneous, and to show facts supporting a new concept. The main facts are as follows: (a) Hydrindantin forms when amino acids react with ninhydrin (Ruhemann (1) ; Abderhalden (2)) ; (b) it reacts with ammonium salts to give Ruhemann’s purple ((1); Harding and Warne-. ford (3) ; Harding Ad MacLean (4)) ; ( c ammonia is formed by ninhydrin ) deamination of amino acids ((1) ; MacFadyen (5)) ; (d) hydrindantin in dilute alkaline solution gives a red color, in concentrated alkaline solution a blue or purple color ((1); Retinger (6)). All these facts have been confirmed by us. The reaction of proline or hydroxyproline with ninhydrin is a special case, giving no purple color (Grassmann and von Arnim (7)), and no ammonia (5). There are three main concepts according to Ruhemann (I), Retinger (6), and Harding et al. (3, 4). In the first the amino acid is oxidatively deaminized by ninhydrin, which is reduced to diketohydrindol; diketohydrindol and ninhydrin condense to form hydrindantin, which then combines with ammonia to give Ruhemann’s purple. In the second, 2 moles of amino acid combine with hydrindantin and the compound splits into two identical nitrogen-free, purple-colored free radicals, analogously to the presumed compound formed from inorganic cations and hydrindantin in strongly alkaline solution. In the third, amino acids are distinguished from amines and ammonia because the former react faster chromogenically with ninhydrin. Amino acids decompose independently of ninhydrin into a glyoxal and ammonia; the glyoxal reduces ninhydrin to diketohydrindol and is oxidized to the corresponding ru-keto acid; ammonia combines with diketohydrindol to form diketohydrindamine, and diketohydrindamine condenses with ninhydrin to form Ruhemann’s purple. The sequence of events is the same in the case of amines and ammonia