Molecular tapes in the structure of isoguaninium chloride

Isoguanine, an analogue of guanine, is of intrinsic interest as a noncanonical nucleobase. The crystal structure of isoguaninium chloride (systematic name: 6-amino-2-oxo-1H,7H-purin-3-ium chloride), C₅H₆N₅O⁺·Cl^-, has been determined by single-crystal X-ray diffraction. Structure analysis was supported by electrostatic interaction energy (E_es) calculations based on charge density reconstructed with the UBDB databank. In the structure, two kinds of molecular tapes are observed, one parallel to (010) and the other parallel to (50-4). The tapes are formed by dimers of isoguaninium cations interacting with chloride anions. E_es analysis indicates that cations in one kind of tape are oriented so as to minimize repulsive electrostatic interactions.