The sequence of deprotonation of pyridine-6-phospho-4-carboxylic acid

The sequence of deprotonation of pyridine-6-phospho-4-carboxylic acid (PPCA) was investigated by employing quantum-chemical calculations performed at the B3LYP/6-311++G∗∗ level of theory. The shape of the potential functions for the proton transition within and between the functional groups of the PPCA was studied for the gas-phase but also the solvent effect approximated by the PCM model was taken into account. Theoretical calculations enabled the evaluation of the minima on the potential energy surface as well as the energy barriers for the proton transition and to estimate whether the barriers are real or apparent i.e., whether the vibrations connected with proton transfer are below or above the appropriate barrier. The obtained data helped to clarify the controversy of deprotonation steps of PPCA that has occurred in the literature.