title: 2-(4-Carb-oxy-piperidinium-1-yl)pyridine-3-carboxyl-ate.
abstract: 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.
authors.affiliation: College of Chemistry, Liaoning University, Shenyang 110036,  People's Republic of China.
authors.forename: Ping
authors.initials: P
authors.lastname: Fan
authors.forename: Chunhua
authors.initials: C
authors.lastname: Ge
authors.forename: Mingjun
authors.initials: M
authors.lastname: Sun
authors.forename: Weiwei
authors.initials: W
authors.lastname: Li
authors.forename: Runshan
authors.initials: R
authors.lastname: Shang