Tautomeric Polymorph of Hypoxanthine from a Tautomeric Amorphous Hypoxanthine Precursor

The nonclassical crystallization processes via amorphous precursors are one of the important principles in the organic biomineralization of guanine. Hypoxanthine is the second component in guanine biominerals. Amorphous hypoxanthine was synthesized for the first time via a neat ball-milling method, and a novel hypoxanthine polymorph (Form IV) was recrystallized from the amorphous hypoxanthine precursor via a moisture-induced solid–solid phase transition. 13C ss-NMR, IR, and Raman spectroscopy confirmed that the amorphous hypoxanthine and Form IV consist of keto-N7H and keto-N9H hypoxanthine tautomers, and it was also found that the short-range order of the amorphous hypoxanthine is more similar to that of Form IV than to the other hypoxanthine polymorphs. The tautomeric-stabilized mechanism of amorphous hypoxanthine provides new insight into the amorphous phases of small organic molecules, and the preparation process of tautomeric Form IV represents a successful application of the organic biomineralization principle of amorphous precursors.