Amino Acids Amino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen. An alpha-amino acid has the generic formula H2NCHRCOOH, where R is an organic substituent; such as hydrogen, a straight or branched chain aliphatic group or aromatic ring or a heterocyclic ring. Ex. Alanine, valine, leucine stc The amino group is attached to the carbon atom immediately adjacent to the carboxylate group (the α–carbon). They are particularly important in biochemistry, where the term usually refers to alpha-amino acids.

Other types of amino acid exist when the amino group is attached to a different carbon atom; for example, gamma-amino acids H2N O OH Gamma-amino-butyric acid In gamma-amino acids (such as gamma-amino-butyric acid) the carbon atom to which the amino group attaches is separated from the carboxylate group by two other carbon atoms. H heterocyclic group

Classification of Amino Acids They are over 300 amino acids occur in nature but 20 amino acids occur only in protein, they classified according to the synthesis by the body into 2 groups. A. According to the synthesis 1. Non essential amino acids- the body can synthesized Alanine, Proline, Asparagine, Aspartic acid , Cysteine, Tyrosine, Serine, Glycine, Glutamine and Glutamic acid 2. Essential amino acids-the body can’t synthesized it. Isoleucine , leucine, Lysine, Methionine, Valine, Phenyl alanine, Threonine,Tryptophan, Histidne and Arginine.

B. Chemical classification of amino acids They are classified according to the side chain into 7 groups:- 1. Aliphatic side chain Alanine, Glycine, Isoleucine, Leucine, Proline and Valine. 2. Aromatic ring Phenyl alanine, Tryptophan and Tyrosine 3. Acidic side chain Aspartic acid and Glutamic acid 4. Amidic ( contain amide group) Asparagine and Glutamine. 5. Sulphur containing Cysteine and Methionine

6. Hydroxylic Serine, Threonine and Tyrosine 7 6. Hydroxylic Serine, Threonine and Tyrosine 7. Basic Arginine, Histidine and Lysine C. Physical classification of amino acids: 1. Hydrophobic Alanine, Isoleucine, Methionine, Phenyl alanine, Proline, Tryptophan, Tyrosine and Valine. 2.Hydrophilic Arginine, Asparagine, Aspartic acid, Cysteine, Glutamic acid, Glutamine, Glycine, Histidine, Lysine, Serine and Threonine.

How does peptide bond form Peptides are formed from amino acid by linking the α-carboxyl (-COOH) group of one amino acid to the α-amino group of another amino acid with the elimination of water. The carbon-nitrogen bond (C-N) thus formed a peptide bond.

Properties of Amino Acid A. Physical properties AA are generally colorless, crystalline solids having melting points above 200 0C They are soluble in water, acid and alkali but insoluble in non-polar organic solvents B. Chemical properties Reaction of AA with alkali:- AA react with alkali to form the corresponding salts. e.g. CH2 NH2 COOH Glycine + CUO CU O CO H2C Cupric glycine

2. Reaction of AA with alcohol:- AAs can be esterified by heating with an alcohol in the presence of anhydrous HCl. CH2 NH2 COOH Glycine + C2H5OH Cl- NH 3+ -CH2 –C-O-C2H5 NH2-CH2 C-O-C2H5 O NaOH HCl, Ethyl α-amine 3. Reaction of AA with formaldehyde:- AAs react with formaldehyde to produce N-methylene amino acid C=O H CH2 COOH Glycine H2N H2C N CH2 COOH + H2O + N-methylene glycine

4. Reaction of AA with Nitrous Acid:- AAs react with nitrous acid (NaNo3+HCl) to form hydroxy acids and nitrogen Glycine CH2 COOH + H2N HONO Nitrous acid COOH +N2 +H2O HO Glycolic acid H+ 5. Reaction of AA with Sanger’s reagent:- AAs react with 2,4,-dinitroflurobenzene also known as Sanger’s reagent to produce yellow colored dinitrophenylamino acids or DNP-amino acids. This reaction is very important in the determination of structure of peptides and proteins and N-terminal amino acid COOH H NO2 +HF CH N R COOH H CH N-H + R F NO2 2,4, DNFB DNFAA (Yellow) α-amino acid

6. Reaction of AA with Ninhydrin:- All α-AAs react with ninhydrin to produce the same purple complex. This reaction is commonly used to test the presence of α-AAs O C OH CH COOH R + NH2 α-amino acid -H2O O Ninhydrin C Purple complex O N O- + RCOOH

Phenyl Iso-thiocyanate 6. Reaction of AA with Edman reagent:- AAs react with phenyl isothiocyanate to produce phenyl thiocarbamoyl which on treatment with mild acidic conditions form a cyclic compound phenyl thiohydantoin. This reaction is used for the detection of N-terminal amino acid. CH COOH R NH2 α-amino acid + SC=N NH-SC-NH H+ CO NH-SC N + H2O Phenyl Iso-thiocyanate Phenyl thiohydantoin Phenyl thiocarbamoyl