[HARDING] THE "NINHYDRIN" REACTION 37 



coloring matter requires the reduction of the triketohydrindene 

 hydrate at some stage or other. 



the reducing action of amino acids and the mechanism of the 

 "ninhydrin" reaction. 



The interaction of alanine and triketohydrindene hydrate results 

 in the oxidation of the amino acid to acetaldehyde and carbon dioxide 

 with the corresponding reduction, and condensation with ammonia, 

 of the triketone. In determinmg the mechanism whereby the amino 

 acid acts as a reducing agent we have had recourse to the dissociation 

 theory of Dakin and Dudley who have shown that all amino acids 

 in dilute solution dissociate into ammonia and the corresponding glyox- 

 al. Thus alanine gives ammonia and methyl glyoxal. 



CH3CHNH2-COOH = CH3COCHO + NH3 



The glyoxals are extremely powerful reducing agents, reducing 

 ammoniacal silver oxide and Fehling's solution in the cold, and it is 

 supposed that methyl glyoxal would reduce the triketone, itself being 

 oxidised to pyruvic acid. 



CH3COCHO + O = CH3-C0C00H 



which by loss of carbon dioxide would yield acetaldehyde. 



CH3COCOOH = CH3CHO + CO2. 



Utilising the theory in this way accounts satisfactorily for the end 

 products of the reaction. The two products of the dissociation of 

 the amino acid would be both utilised^the glyoxal to reduce the 

 "ninhydrin," the ammonia to condense with the reduction product — ■ 

 and thus the dissociation of the amino acid could become complete. 

 The reduction of triketohydrindene hydrate by the glyoxal would 

 give diketo-B-hydrindol. 



C6H4 - CO 



I I 



CO CHOH 



which then combining with a second molecule of the triketone would 

 yield hydrindantin. It appears to us, however, improbable that 

 hydrindantin represents an intermediate stage in the formation of the 



