Physical properties of Amino acids

(a) Amino acids are colourless, crystalline substances comprising sweet taste. They melt along with decomposition at higher temperature (much more than 200°C). They are soluble in water although insoluble in organic solvents.

(b) Apart from glycine, all the α-amino acids are optically active and comprise an asymmetric carbon atom (α-carbon atom). Therefore, each of these acids may present in two optical isomers. In proteins, though, only one isomer of each is generally involved.

(c) Zwitter ion and isoelectric point : as the -NH₂ group is basic and - COOH group is acidic, in neutral solution it presents in an internal ionic form known as a Zwitter ion in which the proton of -COOH group is transferred to the -NH₂ group to make inner salt, as well known as dipolar ion.