The cationic nitrogen of zwitterion 1 is located symmetrically with respect to its intramolecular OHO hydrogen bond. Incorporation of one 18O allows investigation of the H- bond symmetry by the NMR method of isotopic perturbation. In both CD3OD and CD2Cl2 equilibrium isotope shifts are detected at the carboxyl and ipso carbons. Therefore, 1 exists as a pair of interconverting tautomers, not as a single symmetric structure with its ...