ESSENTIAL AMINO ACIDS

Amino acids, as their name implies are compounds that contains both an amino group and a carboxylic acid group. The general structure of amino acid is---

R―CH(NH₂) ―COOH

Where, R is the side chain, it varies from amino acid to amino acid. In this structure of amino acid, the ―NH₂ group is present at α position, located next to ―COOH group. So, it is the general structure of α-amino acids.

The general structure of β-amino acids is---

R―CH(NH₂) ―CH₂―COOH

Where, the ―NH₂ is present at β position.

The general structure of γ-amino acids is---

R―CH(NH₂) ―CH₂―CH₂―COOH

Where, the ―NH₂ group is present at γ position.

The amino acids are particularly important because they are the monomeric units of biologically important polymers called peptides. Proteins are simply large peptides. Peptides and proteins serve many important roles in biological system, e.g., all enzymes are proteins. Hydrolysis of protein by acids, alkalis or enzymes, yield a mixture of amino acids. Mixture of amino acids obtained by the hydrolysis of proteins are mostly α-amino acids.

Twenty α-amino acids, however are known by widely accepted traditional names. These are the amino acids that occurs commonly as constituents of most proteins. The amino acids can be grouped according to the nature of their side chains.

Amino acids with side chain containing H or aliphatic hydrocarbon

Amino acids with side chains containing aromatic groups

Amino acids with side chains containing ―SH, ―SCH₃, or ―OH groups

Amino acids with side chains containing carboxylic acid or amide groups

Amino acids with basic side chains

In addition to the above twenty α-amino acids there are two other α-amino acids

The conversion of cysteine to cystine required additional comment. The ―SH group of cysteine makes a thiol. One property of thiol is that they can be converted to disulphide by mild oxidizing agents.

Configuration of natural amino acids

There are twenty commonly naturally occurring amino acids, called essential amino acids. One is achiral (glycine), all the others have a stereogenic centre. Stereochemical studies of these naturally occurring amino acids have shown that all have the same configuration about the carbon atom carrying the alpha amino group and this configuration is the same as that in L (-) glyceraldehyde. All have the ‘S’ configuration, except (-) cysteine and (-) cystine, which because of the high priority of the sulfur atom gets the stereo descriptor R.

Zwitter ionic structures of amino acids---

An amino acid contains ―NH₂ group, a basic one and ―COOH group, an acidic one. Thus, amino acid exists as Zwitter ion, i.e., a dipolar ion. The Zwitter ionic form of an amino acid is as follows---

NH₃⁺―CH(R)―COO^-

Evidences for the formation of Zwitter ionic structure of amino acid---

1. High melting point of amino acids in comparison to amines or carboxylic acids.

Ph―CO―NH―CH₂―COOH this is N-benzoyl glycine (hippuric acid), its melting is 190⁰C.

The melting point of glycine (H₂N―CH₂―COOH) is 262⁰C.

2. Amino acids are insoluble in polar aprotic solvent such as ether. Most amines and carboxylic acids on the other hand, dissolve in ether. Although the water solubility of the different amino acids varies, all are highly soluble in water.

3. The dipole moment of the amino acids is very large, much larger than those of similar sized molecules with only an amine or a carboxylic acid group.

Dipole moment of propanoic acid (CH₃―CH₂―COOH) is 1.7 D (µ = 1.7 D).

Dipole moment of n-butyl amine (CH₃―CH₂―CH₂―CH₂―NH₂) is 1.4 D (µ = 1.4 D).

Whereas, dipole moment of glycine (H₂N―CH₂―COOH) is 14 D (µ = 14D).

Obviously, a large dipole moment is expected for the molecules that contain a great deal of separated charge.