3-[(2Z)-2-{[4-(2-carboxyethyl)-5-{[(2Z)-3-ethyl-4-methyl-5-oxo-2,5-dihydro-1H-pyrrol-2-ylidene]methyl}-3-methyl-1H-pyrrol-2-yl]methylidene}-5-{[(2E)-3-ethyl-4-methyl-5-oxo-2,5-dihydro-1H-pyrrol-2-ylidene]methyl}-3-methyl-2H-pyrrol-4-yl]propanoic acid