Tautomeric polymorphism of the neuroactive inhibitor kynurenic acid

Kynurenic acid (KYN; systematic name: 4‐hydroxyquinoline‐2‐carboxylic acid, C₁₀H₇NO₃) displays a therapeutic effect in the treatment of some neurological diseases and is used as a broad‐spectrum neuroprotective agent. However, it is understudied with respect to its solid‐state chemistry and only one crystal form (α‐KYN·H₂O) has been reported up to now. Therefore, an attempt to synthesize alternative solid‐state forms of KYN was undertaken and six new species were obtained: five solvates and one salt. One of them is a new polymorph, β‐KYN·H₂O, of the already known KYN monohydrate. All crystal species were further studied by single‐crystal and powder X‐ray diffraction, thermal and spectroscopic methods. In addition to the above methods, differential scanning calorimetry (DSC), in‐situ variable‐temperature powder X‐ray diffraction and Raman microscopy were applied to characterize the phase behaviour of the new forms. All the compounds display a zwitterionic form of KYN and two different enol–keto tautomers are observed depending on the crystallization solvent used.