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), CHNO·Cl, has been determined by single-crystal X-ray diffraction. Structure analysis was supported by electrostatic interaction energy (E) 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 analysis indicates that cations in one kind of tape are oriented so as to minimize repulsive electrostatic interactions.