Biogenic Guanine Crystals Are Solid Solutions of Guanine and Other Purine Metabolites

Abstract

Highly reflective crystals of the nucleotide base guanine are widely distributed in animal coloration and visual systems. Organisms precisely control the morphology and organization of the crystals to optimize different optical effects, but little is known about how this is achieved. Here we examine a fundamental question that has remained unanswered after over 100 years of research on guanine: what are the crystals made of ? Using solution-state and solid-state chemical techniques coupled with structural analysis by powder XRD and solid-state NMR, we compare the purine compositions and the structures of seven biogenic guanine crystals with different crystal morphologies, testing the hypothesis that intracrystalline dopants influence the crystal shape. We find that biogenic “guanine” crystals are not pure crystals but molecular alloys (aka solid solutions and mixed crystals) of guanine, hypoxanthine, and sometimes xanthine. Guanine host crystals occlude homogeneous mixtures of other purines, sometimes in remarkably large amounts (up to 20% of hypoxanthine), without significantly altering the crystal structure of the guanine host. We find no correlation between the biogenic crystal morphology and dopant content and conclude that dopants do not dictate the crystal morphology of the guanine host. The ability of guanine crystals to host other molecules enables animals to build physiologically “cheaper” crystals from mixtures of metabolically available purines, without impeding optical functionality. The exceptional levels of doping in biogenic guanine offer inspiration for the design of mixed molecular crystals that incorporate multiple functionalities in a single material.

The Non‐Classical Crystallization Mechanism of a Composite Biogenic Guanine Crystal

“…At least nine distinct guanine morphologies are known, [ 18 ] including irregular [ 16,19 ] and regular hexagonal plates, [ 5 ] square plates, [ 8 ] irregular polygons, [ 16,20,21 ] and prisms. [ 15,16 ] In these crystals, the thermodynamically disfavoured (100) face (parallel to the high refractive index H‐bonded plane), is preferentially expressed and often oriented toward the incident light, maximizing reflection.…”

“…[13] By precisely controlling the morphology and superstructural organization of the crystals, organisms tune the type of optical effect produced from multilayer inter ference [5,14] and diffuse scattering [6,15,16] to reflection from 3D-photonic crystals. [17] At least nine distinct guanine morphologies are known, [18] including irregular [16,19] and regular hexagonal plates, [5] square plates, [8] irregular polygons, [16,20,21] and prisms. [15,16] In these crystals, the thermodynamically disfavoured (100) face (parallel to the high refractive index H-bonded plane), is preferentially expressed and often oriented toward the incident light, maximizing reflection.…”

“…[14] Almost nothing is known about how organisms generate this striking array of morphologies, which deviate from the thermodynamically stable prismatic morphology in which the (100) face is expressed as a minor facet. [3,22] A recent study ruled out the hypothesis that organisms use dopant molecules to dictate the shape of guanine crystals, [18] leaving the question of morphology control wide open. A prerequisite to understand this conundrum is to obtain a better understanding of the guanine crystallization mechanism.…”

Solid-state separation of hypoxanthine tautomers through a doping strategy

“…The peak at around 123 ppm also appears in the ss-NMR spectrum of the raw pure hypoxanthine that consists of N 9 -hypoxanthine. 41 Considering the above results mentioned, we concluded that the doping forms of hypoxanthine in AG β and AG α are N 7 -hypoxanthine, and in GM are N 9 -hypoxanthine. We propose that guanine and hypoxanthine formed various solid solutions here as biological guanine crystals.…”

Synthesis of porous microplatelets of α form anhydrous guanine in DMSO/water mixed solvents

“…Excess water may not benefit the formation of α-AG crystals consisting of N 7 -guanine because the ratio of N 7 -guanine and N 9 -guanine decreases continuously with the increase of dielectric constant of solvent. 28,29 Hypoxanthine(I) was supposed to be occluded in biological guanine microplatelets mainly composed of β-AG 20,30,31 and the synthetic β-AG twinned crystals. 24,27,32 The influence of hypoxanthine on guanine crystallization was investigated by changing the molar ratios of guanine and hypoxanthine (Fig.…”