2-(4-Carb-oxy-piperidinium-1-yl)pyridine-3-carboxyl-ate.

The title compound, C(12)H(14)N(2)O(4), crystallizes as a zwitterion. A negative charge is delocalized in the deprotonated carboxyl group attached to the pyridine ring. The piperidine N atom accepts a proton and the ring is transformed into a piperidinium cation. There is an intra-molecular N-H⋯O hydrogen bond between the protonated NH and a carboxyl-ate O atom. In the crystal, an O-H⋯O hydrogen bond between the carboxyl group and the carboxyl-ate O atom of another mol-ecule generates a helix along the b axis.