(301d) Tautomeric Self-Inhibition and Modifiers of Guanine Crystal Growth

Nature exhibits a remarkable range of structural properties arising from organic molecular crystals that various organisms employ for communication, camouflage, and thermoregulation. Among these systems, guanine crystals stand out for their vital optical roles. Guanine (C₅H₅N₅O) is a tautomeric compound (i.e., composed of structural isomers) that exists in two primary tautomeric forms in aqueous solutions: N₉-G (~90%) and N₇-G (~10%). Among known polymorphs of guanine, the β-anhydrous form (β-AG) is frequently found in biological systems, typically manifesting as elongated prismatic crystals. Despite the remarkable precision that organisms achieve in tailoring guanine crystal size, shape, and facet orientation, the fundamental mechanisms governing its biomineralization are insufficiently understood. In this presentation, we will discuss how the tautomeric forms of guanine act as modifiers (i.e., native inhibitors), with the major N₉-G tautomer inhibiting crystal growth by putatively interfering with solute incorporation at the crystal – solution growth front. We synthesized β-AG crystals with prominent (100) faces in both aqueous and organic media and employed in situ atomic force microscopy (AFM) to visualize growth in real time. We further used variable-temperature Fourier-transform infrared (FTIR) spectroscopy and density functional theory (DFT) to quantify tautomer distributions and clarify their contributions to growth inhibition. It is common for guanine crystals to incorporate related purines (e.g., hypoxanthine, xanthine, uric acid); therefore, we also investigated how these species function as potential modifiers of β-AG crystallization to influence both bulk morphology and growth kinetics in aqueous solutions. By systematically varying their concentrations and examining the resulting crystals with in situ AFM and scanning electron microscopy (SEM), we identified conditions under which these purine analogues transition from promoters to inhibitors of crystal growth. Collectively, these findings shed new light on guanine biomineralization and underscore the critical roles of tautomeric equilibria and molecular analogues in directing crystal assembly processes.