BIOCHEMISTRY - Textbook - Ostapchenko L. I. - 2012

Chapter 5. STRUCTURE, PROPERTIES, AND FUNCTIONS OF PROTEINS

5.1. Structure and properties of amino acids making up proteins. Peptide bonds

5.1.3. Chemical reactions used for amino acid detection

The ability of Amino Acids to undergo specific reactions is determined by the presence of functional groups in their Structure. Since all Proteinogenic Amino Acids contain an amino group and a carboxyl group attached to the α-carbon atom, they can participate in Chemical Reactions characteristic of all amino acids. Meanwhile, the presence of specific functional groups in the side chains (radicals) of individual amino acids dictates their ability to undergo amino acid-specific reactions.

Ninhydrin test for α-amino acids. The ninhydrin reaction can be used for the detection and quantification of amino acids in solution. This reaction is based on the principle that colorless ninhydrin reacts with an Amino Acid and condenses into a dimer via a nitrogen atom cleaved from The amino acid's α-amino group. This results in The formation of a red-purple pigment. Simultaneously, decarboxylation of the amino acid occurs, yielding CO2 and the corresponding aldehyde. The ninhydrin reaction is widely applied in protein Primary Structure studies:

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Since the color intensity is proportional to The amount of amino acids in the solution, this reaction is used to determine the concentration of α-amino acids.

Specific Reactions for individual amino acids. Qualitative and Quantitative determination of specific amino acids is made possible by the presence of distinct functional groups in their side chains. Arginine is detected using a qualitative test for the guanidino group (the Sakaguchi reaction), whereas Cysteine is identified by the Folin test, which is specific for the SH-group of this amino acid. The presence of aromatic amino acids in solution is determined by the xanthoproteic reaction (nitration reaction), and the presence of a hydroxyl group in the aromatic ring of Tyrosine is detected using Millon's reagent.