Abstract As a model system for the internal and external aldimines of the coenzyme pyridoxal phosphate (PLP) in PLP dependent enzymes we have studied the 1H and 15N NMR spectra of the 15N labeled Schiff base 3-carboxy-5-methyl-salicylidenaniline ( 1) dissolved in CD 2Cl 2. 1 contains a charge relay system with two strongly coupled intramolecular hydrogen bonds of the OHOHN type. One-bond 15N 1H scalar spin–spin coupling constants and chemical shifts of partially deuterated 1 were measured in the temperature range between 243 and 183 K and analyzed assuming an exchange between three tautomeric states exhibiting well defined hydrogen bond geometries. The analysis shows that the dominant structure 1b corresponds to the zwitterion O H⋯O −⋯H N +, where deuteration of one bond leads to a shortening of the other. This anti-cooperative effect is revealed by the vicinal isotope effects on the proton chemical shifts. By contrast, forms 1a and 1c are characterized by the structures O H⋯O H⋯N and O −⋯H O⋯H N +, correspondingly, whose hydrogen bonds exhibit a cooperative coupling. We predict that 1a will dominate at high temperatures and low dielectric constants, whereas 1c will dominate at low temperatures and large dielectric constants. The comparison with model systems which do not contain the additional COOH-group indicates that the latter is responsible for the dominance of the zwitterionic structure of the OHN hydrogen bond. The implications of these findings for the function of the coenzyme pyridoxal phosphate in its natural environment are discussed.