3-[2-[(E)-[3-(2-carboxyethyl)-5-[(4-ethenyl-3-methyl-5-oxo-1,2-dihydropyrrol-2-yl)methyl]-4-methylpyrrol-2-ylidene]methyl]-5-[(3-ethyl-4-methyl-5-oxo-1,2-dihydropyrrol-2-yl)methyl]-4-methyl-1H-pyrrol-3-yl]propanoic acid