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White to off-white to yellow solid

5-Hydroxynicotinic acid is a nicotinic acid analog with potential inhibitory effect on the metabolism nicotinic acid by human platelets. Used in the investigation of the hydroxylation mechanism of the flaovoprotein 2-methyl-3-hydroxypyridine-5-carboxylic acid oxygenase (MHPCO).

ChEBI: 5-Hydroxynicotinic acid is an aromatic carboxylic acid and a member of pyridines.

5-hydroxynicotinic acid (5HNA) could be used as a model the following questions were addressed: (i) is it possible to promote the crystallization of a tautomeric form dominant in a specific solvent through solvate formation? (ii) Does that form persist if the memory of solvation is erased through thermal desolvation?
A thermodynamic analysis based on DSC and Calvet drop microcalorimetry results allowed us to rationalize these observations, indicating that (i) 5HNA·H2O is predicted to spontaneously lose water, even for a relative humidity of 100%, hence its robustness is most certainly of kinetical origin; (ii) 5HNA·DMSO is thermodynamically stable when a saturation DMSO pressure can be established over the sample, but becomes unstable when exposed to an atmosphere where the solvent is absent. The kinetically easier desolvation of 5HNA·DMSO compared to 5HNA·H2O may be related to the fact that water is isolated in the crystal lattice (isolated site hydrate) while DMSO is placed in channels (channel solvate)[1].

[1] Joseph, Abhinav et al. “Tautomer selection through solvate formation: the case of 5-hydroxynicotinic acid?.” CrystEngComm 13 (2019): 2220–2233.