Biogenic twinned microplatelets of the anhydrous guanine (AG) β form have been found in copepod cuticles and scallop eyes, which function as multilayer reflectors. In this work, twinned AG β microplatelets with morphologies such as cross-shape and square-shape were synthesized for the first time in the presence of a polyvinylpyrrolidone additive in formamide and water. The twinning angle of the two c diffraction vectors (84°) for the synthetic twinned guanine crystals is very similar to that of biogenic twinned guanine microplatelets observed in scallop eyes. The angles between the two observed c diffraction vectors of the synthetic twinned AG β form microplatelets were about 84° and 20°, while a twinning angle of 84° was observed in the biogenic twinned guanine microplatelets exposing the (100) face. We proposed a formation mechanism for the twinned guanine microplatelet crystals. The formation of the twinned crystals with angles of 84° and 20° between the c diffraction vectors might be related to the attachment of G-quartet molecular assemblies on the guanine plate-like crystals with a certain angle at the early stage of crystallization.

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.

Guanine crystals with unique optical properties in organisms have been extensively studied and the biomineralization principles of guanine are being established. This review summarizes the fundamental physicochemical properties (solubility, tautomers, bands, and refractivity), polymorphs, morphology of biological and synthetic forms, and the reported biomineralization principles of guanine (selective recrystallization of amorphous precursor, preassembled scaffolds, additives, twinning, hypoxanthine doping, fluorescence, and assembly). The biomineralization principles of guanine will be helpful for the synthesis of guanine crystals with excellent properties and the design of functional organic materials for drugs, dyes, organic semiconductors, etc.

The solid-state separation of hypoxanthine tautomers was realized by a doping strategy. The doping forms of hypoxanthine in HAmG, AG β and AG α are N ₇ -hypoxanthine, and in GM and dehydrated-GM are N ₉ -hypoxanthine.

The organic additives have a strong influence for the dissolution process of calcite {104} planes. We find that there are three kinds of additives and four dissolution modes of calcite {104} planes while changing the additive concentrations in the solution.