Chemical Reactions of Amino Acids:

You may ask: what is the relevance of chemical reactions in the study of biological phenomena? Let us consider few of such reasons. First, taking advantage of chemical reactivity of certain amino acids has permitted chemical modification of proteins. Secondly, qualitative detection and quantitative measurement of amino acids and proteins were possible because scientists have taken advantage of coloured solution formed when these acids are made to react with certain chemical reagents. Another very important reason is that the biological function of proteins is known to be closely linked with the chemical behavior of their R groups.

Reactions of α-Carboxyl Group:

The α-carboxyl group can react with ammonia and primary amines to yield unsubstituted and substituted amides respectively.

i)

ii)

iii)

Reactions of α-amino Group:

i) Free α- amino groups of amino acids may react with aldehydes to form Schiff bases.

ii) Acylation: amino groups can be acylated with acid anhydrides and acid halides.

iii) Ninhydrin Reaction

Amino acids, through amino groups can react with ninhydrin (or triketohydrindene hydrate), the very strong oxidizing agent. This agent oxidatively deaminate α- amino functional group of amino acids. Products of the reaction are carbon dioxide, aldehyde, ammonia, and hydrindantin.

The significance of the reaction is in its application in spectrophotometric quantification of amino acids at 570 nm. [α-imino acids, example Proline and Hydroxyproline, yield yellow complex instead of purple, with absorption maxima at 440nm].

Reactions of Specific Functional Groups of Side Chain:

Specific reaction of amino acids is significant for sequencing, degradation and chemical synthesis of peptides and proteins. These reactions are helpful in recognizing specific functional groups and therefore specific amino acids having such functional groups.

i) Reactions of Sulfyhydryl Group:

a) Disulfide bond (bridge) formation

Thiol groups of cysteine can react with one another to create disulfide species.

b) Reactions of cysteine with iodoacetic acid.

Cysteine is also known to react with iodoacetic acid to yield S-carboxymethyl cysteine derivatives.

ii) Xanthoproteic Reactions of Aromatic Ring:

Amino acids with aromatic nuclei create yellow nitro derivatives on heating with concentrated nitric acid. This reaction is based on nitration of benzene ring with concentrated HNO₃ yielding yellow derivatives of nitrobenzenes. Reaction is provided by tyrosine, tryptophan, and polypeptide that have these amino acids. Phenylalanine is more difficult to nitrate and so needs H₂SO₄ as a catalyst.

iii) Millon's Reaction:

Amino acid called tyrosine, a hydroxybenzene-radical containing compound, reacts with Millon's reagent (a solution of mercuric and mercurous ions in nitric and nitrous acids) to form red complexes.

iv) Reaction of Tryptophan with Formaldehyde:

Tryptophan reacts with formaldehyde reagent in presence of concentrated H₂SO₄, to create a bluish- purple compound appearing as ring at interface of liquids. This reaction is characteristic of indole- ring of tryptophan.

v) Sakaguchi Reaction:

Arginine, an amino acid containing guanidinium group, reacts with α- naphthol and oxidizing agent, like bromine water, to provide a red color compound.

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