Towards a detailed description of pyridoxamine tautomeric species

3-Hydroxypyridine (3-HPY) and pyridoxamine (PM), one of the most relevant biological and biomedical vitamin B-6 derivatives, are characterized to possess several ionic groups. This allows a complex tautomeric and ionic equilibria network formation that makes these systems difficult to characterize. Little is actually known about the physico-chemical descriptors of the minor tautomers which are essential for the biological and biomedical activity of PM. To characterize them, we have studied theoretically the ionic and tautomeric forms of 3-HPY and PM. Geometry, charges, molecular electrostatic potential maps and free energy values have been computed by using the PCM-MP2/6-311++ G(d,p) theoretical level, whereas excitation energies and 13 C chemical shifts have been computed on the optimized geometries at the PCM-B3LYP/6-311++ G(d, p) level. Theoretical C-13 chemical shifts and excitation energies have been validated experimentally. The individual protonation effect of the phenol group and pyridinic nitrogen as well as their combined effect on the depicted indicators is discussed. The provided results represent a further step in understanding the tautomeric equilibria in PM and enhance the characterization of the minor but essential tautomers. It also constitutes a plausible model to apply in the study of negligible tautomers in different systems.