3-(2-{[3-(2-carboxyethyl)-5-[(3-ethenyl-4-methyl-5-oxo-1H-pyrrol-2-ylidene)methyl]-4-methyl-1H-pyrrol-2-yl]methyl}-5-[(4-ethenyl-3-methyl-5-oxo-1H-pyrrol-2-ylidene)methyl]-4-methyl-1H-pyrrol-3-yl)propanoic acid