Guanine Crystal Formation at Physiological pH

Guanine crystals are the principal component of many biocrystals with optical functions. Typically exhibiting unique morphologies and being metastable β anhydrous guanine (β-AG) rather than the thermodynamically stable α anhydrous polymorph (α-AG), many questions remain regarding the mechanisms by which organisms control their formation. However, efforts to elucidate these using bio-inspired approaches have been limited by the very low solubility of guanine in aqueous solutions at physiological pH. Here, we demonstrate an enzymatic approach based on the purine metabolism process that yields significant quantities of guanine crystals in aqueous solution at neutral pH. Significantly, this mirrors processes believed to generate guanine crystals in vivo. The enzyme purine nucleoside phosphorylase (PNP) is used to continuously convert guanosine to guanine and generate supersaturation, and pure β-AG or α-AG can be produced by changing the reagent concentrations or introducing stirring. We also show that the rate of change of supersaturation is crucial in determining the polymorph, demonstrating that organisms can generate β-AG crystals by simply controlling the crystallization conditions. This work bridges the gap between in vitro and biological crystallization and provides a facile means of studying the crystallization of biological molecules and ultimately generating functional materials using sustainable processes.