3-[2-[[3-(2-carboxyethyl)-5-[(Z)-(4-ethyl-3-methyl-5-oxo-pyrrol-2-ylidene)methyl]-4-methyl-pyrrol-2-ylidene]methyl]-5-[(3-ethyl-4-methyl-5-oxo-pyrrol-2-ylidene)methyl]-4-methyl-1H-pyrrol-3-yl]propanoic acid