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Classification Of Amino Acids

All the naturally occurring amino acids are classified on the basis of their R groups or side chains : (a) amino acids with non –polar R groups, (b) amino acids with polar but unchanged R groups (C)
Amino acids with positively charged R groups and (d) amino acids with negatively charged R groups. The classification of amino acids along with the trivial names as used in protein chemistry.

Zwitterionic Nature of Amino Acids

Amino acids contain acidic and basic groups and can both donate and accept protons, hence they are said to be amphoteric. The carboxyl group is acidic and can donate a proton by dissociation. The amino group, being  basic, can accept a proton. When the groups are ionized, the amino acid is said to be zwitterion. Amino acid in aqueous solution are in zwitterionic form.


Amino acids an Electrolytes

The α-amino acids in aqueous solution exists as dipolar ions, but the ionic state of the molecules determined by the ph of the medium in which it exists. Consider the simplest amino acid , glycine, which consists of none carboxyl group and one amino group due to which is has the characteristic acid- base properties . This property is shown by many α- amino acids in solution and so they behave as electrolytes .glycine can react with acid and alkali when dissolved in water and assume a dipolar state. The pH at which the dipolar (zwiitterion) does not migrate in an electrical field is called the isoeletric pH (pHI). At this pH the amino acid is electrically neutral since the number of positive charges equal the negative charges. The isoeletric pH of amino acids can be found out by acid base titrations.

Amino acids are ionized to various degrees depending upon the ionizable groups they possess. The dissociable constant, pK values, are determined for the ionizable groups. If a solution of glycine hydrochloride (0.2 M glycine solution with 0.2 m HCT is titrated with 0.1 M NaOH solution and the pH be determined. The first pK1 value is obtained when the COOH group is ionized and the second pK2 value is observed when ammonium ion is dissociated. The result can be graphically plotted in the form of a biphasic curve.

three ionizable states of glycine

Titration curve of amino acid glycine showing values Isoeletic

UV absorption spectra of aromatic amino acids

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