an aromatic amino acid selected from {HEAD ~ the class} consisting of anthranilic acid{TAIL ~ , p aminobenzoic acid}, m aminobenzoic acid, 3 amino p toluic acid, 3 aminosalicylic acid, 3 amino 4 methoxybenzoic acid, 6 amino m toluic acid, 3 amino 4 chlorobenzoic acid, 2 amino 5 nitrobenzoic acid, 2 nitro 5 aminobenzoic acid, 5 aminoisophthalic acid 