Amino acids are colourless, crystalline solids. They are water-soluble high melting solids and behave like salts. The a-carbon atom has 'R' which is a side chain. This side chain is different for different amino acids. The carboxyl and amino groups interact resulting in the transfer of proton from carboxyl group to amino group. Thus the amino acid exists in ionised form known as zwitter ion. This explains the physical properties of these a-amino acids.
In aqueous solution, the carboxyl group can loose a proton and amino group can accept a proton giving rise to a dipolar ion which is neutral but has both positive and negative charges.
In zwitter ionic form, amino acids slow amphoteric behavior as they react both with acids and bases. In acidic solution, the carboxylate function (- COO-) accepts a proton and gets converted to carboxyl substituent (- COOH) while in basic solution the
ammonium substituent (+NH3) changes to amino group (-NH2) by losing a proton.
In acidic solution, an amino acid exists as a positive ion and migrates towards the cathode in an electric field, while in alkaline solution it exists as a negative ion and migrates towards anode. At a certain hydrogen ion concentration (pH), the dipolar ion exists as a neutral ion and does not migrate to either electrode. This pH is known as the isoelectric point of the amino acid.
