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Amino acids and Proteins:
Amino acids are derivatives of carboxylic acids in which a hydrogen atom at the α-carbon is replaced by an amino group. Amino acids are obtained by hydrolysis of proteins. The amino acids can be represented by
About twenty five amino acids are obtained from proteins, of which about ten are essential, i.e. a deficiency in any one prevents growth in young animals and may even cause death. Simplest amino acids are
R —— C —— COOH CH3 —— C —— COOH
Alanine and all amino acids except glycine contain an asymmetric carbon atom and exhibit optical isomerism (naturally occurring amino acids are laevorotatory). Amino acids contain amino and carboxyl groups and show the properties of both groups. Amino acids have high melting points and are usually water soluble. They usually exist as inner salts or zwitter ions or dipolar ion (a species containing both positive and negative charges).
Amino acids are amphoteric.
Amino acids are linked together by peptide linkages.
Peptides are condensation products of two or more amino acids through their amino and carboxylic groups involving elimination of water molecules. Amino acid polymers with relative molecular masses 10,000 or less are called polypeptides. Those with larger molecular masses are called proteins e.g. egg albumen 40,000.
Illustration . Explain why does glycine exist as a Zwitter ion but anthranilic acid does not.
Solution: The ¾COOH group of glycine releases H+ ion which is accepted by ¾NH2 group. Thus glycine exists as a zwitter ion.
However, the electron withdrawing nature of —COOH group reduces electron density on N in anthranilic acid. Thus nitrogen atom of amino group fails to accepts the proton released by the —COOH group. That is why anthranilic acid does not exist as zwitter ion.