an aromatic amino acid selected from the class consisting of anthranilic acid{HEAD ~ , 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, {TAIL ~ 2 amino} 5 nitrobenzoic acid, 2 nitro 5 aminobenzoic acid, 5 aminoisophthalic acid 