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Amino acids as dipolar ions

Although the amino acids are commonly shown as containing an amino group and a carboxyl group, H2NCHRCOOH, certain properties (both physical and chemical) are not consistent with this structure:
  • In contrast to amines and carboxylic acids, the amino acids are non-volatile crystalline solids, which melt with decomposition at fairly high temperatures.
  • They are insoluble in non-polar solvents such as petroleum ether, benzene, or ether and are appreciably soluble in water.
  • Their aqueous solutions behave like solutions of substances of high dipole moment.
  • Acidity and basicity constants are ridiculously low for –COOH and –NH2 groups. Glycine, for example, has Ka = 1.6 × 10–10 and Kb = 2.5 × 10–12, whereas most carboxylic acids have Ka values of about 10–5 and most aliphatic amines have Kb values of about 10–4.
All these properties are quite consistent with a dipolar ion structure for the amino acids (I).
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Isoelectric point of amino acids

The hydrogen ion concentration of the solution in which a particular amino acid does not migrate under the influence of an electric field is called the isoelectric point (pI) of that amino acid. The isoelectric point (pI) is the pH at which the amino acid exists only as a dipolar ion with a net charge zero.
At isoelectric point, for a neutral amino acid, pI = Description: 49851.png.

Preparation of amino acids

  1. Amination of α-halo acids
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  2. From diethyl malonate
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  3. Gabriel phthalimide synthesis
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  4. Reductive amination
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  5. Strecker’s synthesis
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  6. Reactions of amino acid: Amino acids are detected by ninhydrin test. All amino acids give violet-colored product with ninhydrin (triketohydroindene hydrate), except proline and 4-hyroxy proline, which gives yellow color with it.
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    Heating of α-amino acids leads to dehydration intramolecularly to form cyclic diamides. When alanine is heated, then two diastereomers are obtained. One of them (trans) is not resolvable.
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    When β-amino acids are heated, α,β-unsaturated salt are formed.
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    γ, δ, ε-Amino acids when heated alone gives γ, δ-lactam, and polymer, respectively.
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A peptide is an amide formed by intermolecular reaction of the amino group of one amino acid and the carboxyl group of the second amino acid. Dipeptides are made from two amino acids, tripeptides are made from three amino acids, and so on. The amino acids may be the same or different. If there are 4–10 amino acid residues, the peptide is called an oligopeptide. A polypeptide is a chain made up of many amino acids. The terms peptide and polypeptide are often used interchangeably. A protein consists of one or more polypeptide chains and each chain can contain as many as several hundered amino acids. The total number of residues may vary from 50 to over 1000.

By convention, the amino acid with the free amino group (N-terminus) is written at the left end and the one with the unreacted carboxyl group (C-terminus) at the right end. The suffix -ine is replaced by -yl for each amino acid in the chain reading from left to right, followed by the full name of the C-terminal amino acid.


Proteins are categorized according to (a) shape and (b) their biological function. Proteins according to shape are further classified as globular, somewhat spherical and fibrous, long fibers or planar sheets. According to their biological action, they are classified as enzymes, hormones, antibodies, etc. A protein has secondary, tertiary, and quaternary structures.
The primary structure is simply the amino acid sequence of the peptide chain. The secondary structure is a result of the different conformations that the chain can take. The tertiary structure is determined by any folding of the chain in on itself. A quaternary structure results when two or more peptide chains in some proteins are linked together by weak forces of attraction of their surface groups. Such proteins are called oligomers (dimers, trimmers, etc.).
Protein found in living system with definite configuration and biological activity is termed as native protein. If a native protein is subjected to physical or chemical treatment, which may disrupt its higher structures (conformations) without affecting its primary structure, the protein is said to be denatured. During denaturation, the protein molecule uncoils from an ordered and specific conformation into a more random conformation, leading to precipitation. Thus denaturation leads to increase in entropy and loss of biological activity of the protein. The denaturation may be reversible or reversible. Thus, the coagulation of egg white on boiling of egg protein is an example of irreversible protein denaturation. However, in certain cases it is found that if the disruptive agent is removed, the protein recovers its original physical and chemical properties and biological activity. The reverse of denaturation is known as renaturation.

Tests of proteins

  • Biuret test: Addition of a very dilute solution of copper sulphate to an alkaline solution of a protein produces a red or violet color.
  • Million’s test: When a solution of mercuric nitrate containing nitrous acid is added to a protein solution, a white precipitate is formed which slowly turns pink.
  • Xanthoprotic test: Proteins produce a yellow color when treated with concentrated HNO3.

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