Chelidamic acid monohydrate: the proton complex of a multidentate ligand

Chelidamic acid, 4‐hydroxy­pyridine‐2,6‐di­carboxyl­ic acid, is found to be zwitterionic in its solid monohydrate form, C₇H₅NO₅·H₂O, with the aryl­oxide and one carboxyl­ate group remaining protonated, but the other carboxyl­ate group losing its proton to the pyridine N atom. In this, it is unlike its parent, dipicolinic acid (pyridine‐2,6‐di­carboxyl­ic acid), which also crystallizes 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 component of an extended hydrogen‐bonded network.