Amino acids are classified as acidic, basic or neutral depending upon the relative number of amino and carboxyl groups in their molecule. Equal number of amino and carboxyl groups makes it neutral, e.g. valine, glycine, etc.; more number of amino than carboxyl groups makes it basic e.g. lysine, arginine and histidine; and more carboxyl groups as compared to amino groups makes it acidic e.g. aspartic acid and glutamic acid. If the side chain comprises of aromatic ring, it is called aromatic amino acid e.g. phenylalanine, tyrosine and tryptophan.