Chelidamic acid monohydrate: the proton complex of a multidentate ligand

Chelidamic acid, 4-hydroxypyridine-2,6-dicarboxylic acid, is found to be zw itterionic in its solid monohydrate form, C7H5NO5. H2O, with the aryloxide and one carboxylate group remaining protonated, but the other carboxylate g roup losing its proton to the pyridine N atom. In this, it is unlike its pa rent, dipicolinic acid (pyridine-2,6-dicarboxylic acid), which also crystal lizes as a monohydrate, but one in which the acidic H atoms remain bound to the carboxylate groups. In both structures the water molecule is a compone nt of an extended hydrogen-bonded network.