Investigation on the formation mechanism of twinned crystals of hypoxanthine-doped beta-phase anhydrous guanine microplatelets

It is well known that guanine crystals, particularly those in the form of microplatelets, are widely used in a variety of bio-optical structures. However, it is a big challenge to realize the controlled crystallization of guanine with defined size, morphology, crystal phase and orientation. Herein, hypoxanthine-doped β phase anhydrous guanine (I-doped β-AG) microplatelet crystals with varying hypoxanthine contents were obtained in the presence of a polymer additive poly(1-vinylpyrrolidone-co-vinyl acetate) in formamide. The hypoxanthine content of the I-doped β-AG can be controlled from 0 to 29 mol%. The obtained twinned crystalline square-like and hexagon-like I-doped β-AG microplatelets are similar to the square-like and hexagon-like biogenic twinned guanine microplates found in scallops and copepods. A possible formation mechanism for the square-like twinned guanine microplatelets was proposed as follows. A thin single crystalline β-AG nanoplate layer exposing the (100) face first forms at the very early stage and a G-quartet (G4) assembly attaches to the (100) face of β-AG via π-π interaction in a certain direction. A second β-AG nanoplate layer nucleates and grows along the c2 axis about 84° from the c1 axis of the first β-AG crystal layer. The occlusion of hypoxanthine in β-AG might be a key factor for the attachment of a G4 assembly on the (100) face of β-AG and the formation of twinned β-AG microplatelet crystals. This work may shed light on the formation mechanism of biogenic twinned guanine crystals.