Chapter 7. Amino acids and their derivatives

Aliphatic amino acids contain a carboxyl group and an aliphatic amino group in their structure. Formulae of amino acid drugs are presented in the Table 7.1.

Table 7.1. Amino acids and their derivatives

Compound name (INN, English, Latin) Chemical structure	Physical and chemical properties Application
Glutamic acid Acidum glutaminicum C5H9N04 M.m. 147.13	A white or almost white crystalline powder or colorless crystals, freely soluble in boiling water, slightly soluble in cold water, practically insoluble in acetic acid, in acetone and in alcohol
gamma-Aminobutyric acid (GABA, aminalon) Aminalonum C4H9NO2 M.m. 103.12	White crystalline powder with a faint characteristic odor; easily soluble in water
Cysteine Cysteinum C3H7NO2S M.m. 121.16	L-cysteine White crystalline powder with a faint characteristic odor; soluble in water, diluted sulfuric and hydrochloric acids
Compound name (INN, English, Latin) Chemical structure	Physical and chemical properties Application
Methionine Methioninum C5H11NO2S M.M. 149.21	White crystalline powder with a characteristic odor; difficultly soluble in water, easily soluble in diluted mineral acids, caustic alkali and ammonia solutions
Piracetam Pyracetamum C6H10N2O2 M.M. 142.16	2-oxo-1-pyrrolidine acetamide White or almost white crystalline powder. It shows polymorphism. Easily soluble in water

PHYSICAL PROPERTIES

Amino acid drugs are usually white crystalline powders with a faint characteristic odor. They are soluble in water due to inner salt formation.

The State Pharmacopeia of the Russian Federation requires specific optical rotation determination in optically active drugs. Methionine is a racemate.

Glutamic acid and piracetam identification include IR spectroscopy.

CHEMICAL PROPERTIES

Acid-base properties

Complexation reactions with Ag⁺, Cu²⁺, Hg²⁺, Co²⁺ salts are the result of amino acids acidic properties. The group reaction is copper salt formation. Amino acid is preliminary converted into sodium salt, avoiding alkali excess:

Ninhydrin Test

Ninhydrin reaction is a common reaction for all amino acids. Ninhydrin is a stable hydrate of 1,2,3-trioxo-hydrindan:

Both equilibrium forms react with amino acids:

SPECIFIC REACTIONS

Glutamic acid

Dicarboxylic glutamic acid is dehydrated with pyrrolidone carboxylic acid formation after heating:

The obtained pyrrolidone carboxylic acid condensates with resorcine. A product has a red-violet color with green fluorescence in ammonia solution:

GABA (Aminalon)

Basic properties of amino acids disappear after amino group blocking by the aldehydes. NaOH solution is added to aminalon until phenolphthalein gives a pink color. The color vanishes when formaldehyde is added to this solution:

The pink color vanishes as formaldehyde eliminates basic properties of amino group.

Methionine

The strong odor of mercaptans appears when methionine reacts with 30% alkali and after the subsequent acidifying:

Ammonia is released after heating as a result of a reaction between amino acid drug and strong alkali solution. Ammonia vapor colors filter paper soaked in sodium nitroprusside in red-violet color.

Cysteine

Cysteine releases sodium sulfide when heated with alkali. Sodium sulfide has a strong odor and causes blackening of paper soaked in lead acetate:

ASSAY METHODS

Kjeldahl nitrogen assay

The measurement is carried out in several stages in a nitrogen analyzer.

At the first stage amino acid is mineralized by heating with concentrated sulfuric acid in the presence of catalysts (Cu²⁺, Hg²⁺ salts, Se);

At the second stage alkali excess is added to the reaction medium and the released ammonia is removed by steam distillation to the receiving vessel with boric acid:

NH₄HSO₄ + 2NaOH = Na₂SO₄ + NH₃↑ + 2H₂O

Boric acid captures ammonia with ammonium tetrahydroborate formation:

B(OH)₃ + H₂O = H[B(OH)₄]

NH₃ + H[B(OH)₄] = NH₄[B(OH)₄]

At the third stage ammonium tetrahydroborate is titrated with standard solution of hydrochloric acid:

NH₄[B(OH)₄]+ HCl = NH₄Cl + H₃BO₃ + H₂O

Carry out a blank titration.

Alkalimetry

GABA (Aminalon) and other amino acids, that have one carboxyl group, can be titrated with alkali provided the preliminary blocking of amino group by formaldehyde (formol titration):

Glutamic acid can be determined by one carboxyl group:

Glutamic acid can be determined by two carboxyl groups, using formol titration.

Acid-base titration in nonaqueous medium

Amino acids are dissolved in glacial acetic acid and then titrated as bases. In the medium of protogenic solvent drug (in this case, aminalon) accepts proton: