Amino acids can exist as L-isomers or D-isomers. However, with respect to the conformational stability, all amino acids are usually found in the L-isomeric form.
Illustration: Isomerism in amino acids
All amino acids except glycine contain an asymmetric centre that makes them chiral in nature due to which they can rotate the plane of polarized light. The two enantiomers are designated as D and L, which rotate the plane of polarization in opposite directions. The two enantiomers of amino acids are non-superimposable mirror image due to the spatial arrangement of four different groups about the chiral carbon atom. Rotation of either isomer about its central axis will not give rise to the other isomeric structure.
Illustration: Acid-base properties of amino acids
Amino acids in acidic medium exist in the completely protonated form carrying a net positive charge, which can be confirmed by means of simple paper electrophoresis. The sample solution is applied at the centre of the strip and current is passed through it. The colorless amino acid solution can be detected by spraying the strip with ninhydrin, which gives it a purple color. Migration of the spot towards the negatively charged cathode confirms the net positive charge of the amino acid.
All amino acids exhibit a characteristic titration curve with distinct pK values. 0.1N NaOH is added to the acidic amino acid solution. The cationic form of the amino acid is gradually converted into its neutral or zwitterionic form by loss of a proton from its COOH group. This can again be confirmed by electrophoresis where there is no migration of the sample spot. Number of equivalents of alkali being consumed is plotted against the pH of the amino acid solution to obtain the titration curve. pK1 of glycine is found to be 2.34 i.e. it starts to lose its carboxyl group proton at this pH. Removal of the proton from the amino group constitutes the second stage of the titration curve. Continued addition of alkali to the amino acid solution gradually converts the zwitterionic form into the anionic form. Migration of the sample spot towards the anode during electrophoresis confirms this. The pK2 of an amino acid is obtained by continued addition of alkali to the neutral solution of the amino acid. pK2 of glycine is found to be 9.6. Some amino acids having positively or negatively charged side chains will have pK1, pK2 and pKR, which corresponds to ionization of the side chain. These amino acids have good buffering capacity around 1 pH unit on either side of their pK values.
