(1E,2S)-1-{(2E)-2-[(2S)-2,6-dicarboxy-2,3-dihydropyridin-4(1H)-ylidene]ethylidene}-5-{[2-O-(beta-D-glucopyranosyl)-beta-D-glucopyranosyl]oxy}-6-hydroxy-2,3-dihydro-1H-indolium-2-carboxylate