X-ray crystal structures of pyrazinic acid 1 and isomeric methylpyrazine carboxylic acids 2-4 are analyzed to examine the occurrence of carboxylic acid-pyridine supramolecular synthon V in these heterocyclic acids. Synthon V, assembled by (carboxyl) OH⊙⊙⊙ N (pyridine) and (pyridine) CH⊙⊙⊙ O (carbonyl) hydrogen bonds, controls self-assembly in the crystal structures of pyridine and pyrazine monocarboxylic acids. The recurrence of acid-pyridine ...