Three polymorphs of hypoxanthine obtained by evaporation from three distinct solvents

Abstract

Hypoxanthine is an important component in guanine biominerals, and its polymorphs and morphology controls are rarely reported. In this work, three polymorphs of hypoxanthine were obtained by a solvent evaporation method: form I (triclinic) from an aqueous solution, form II (monoclinic) from an ethanol solution, and form III (a newly proposed polymorph but not observed first) from an ammonia solution. Solubility measurements in water and ethanol indicated that the stability order of the three polymorphs of hypoxanthine was form I > form III > form II. The Raman, IR and ¹³C ss-NMR spectra indicated that the crystal structures of forms I and III were similar but different from that of form II. The formation of form II was related to the hypoxanthine assemblies in ethanol, as demonstrated by UV spectroscopy. The PXRD patterns and SEM images showed that the three polymorphs had different growth behaviors, as revealed by comparing the hydrogen-bonding planes and π–π stacking modes. Form I had a flat hydrogen-bonding plane with non-vertical π–π stacking, benefitting v_{∥hydrogen-bonding}, while form II had a wavy hydrogen-bonding plane with vertical π–π stacking, benefitting from v_{∥π–π stacking}. The polymorph and morphology controls of hypoxanthine can extend the understanding of the principles of organic biomineralization based on guanine biominerals.