Ninhydrin Synthesis

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Ninhydrin test is performed to detect the presence of free α-amino group (-NH2) which presents in all amino acids, proteins or peptides. It is an endothermic process involving redox reaction. Ninhydrin is a powerful oxidizing agent which also known as triketohydrindene hydrate. First, an oxidative deamination reaction occurs as the α-amino acid reacts with ninhydrin. Two hydrogens from the α-amino acid are elicited to produce an alpha-imino acid. On the same time, the ninhydrin itself undergoes reduction by losing an oxygen atom to form reduced ninhydrin, hydrindantin. Next, hydrolysis reaction happens. The amine group in the alpha-imino acid reacts with the water molecule to form an alpha-keto acid with an ammonia molecule. The alpha-keto acid then undergoes decarboxylation to form an aldehyde with a carboxyl group (CO2). The net result includes hydrindantin, aldehyde, ammonia, and CO2. The hydrindantin and ammonia produced are responsible for the colour formation. The process is continuing as ninhydrin condenses with ammonia and hydrindantin to produce an intensely blue or purple pigment, Ruhemann's purple. This reaction provides an extremely sensitive test for amino acids. Ninhydrin which is originally yellow reacts with amino acid and turns deep purple. The colour intensity produced is directly proportional to the amino acid

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