3-[2-[3-(2-Carboxy-ethyl)-4-methyl-5-(3-methyl-5-oxo-4-vinyl-1,5-dihydro-pyrrol-2-ylidenemethyl)-1H-pyrrol-2-ylmethyl]-4-methyl-5-(4-methyl-5-oxo-3-vinyl-1,5-dihydro-pyrrol-2-ylidenemethyl)-1H-pyrrol-3-yl]-propionic acid