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.
Spectacular colors and visual phenomena in animals are produced by light interference from highly reflective guanine crystals. Little is known about how organisms regulate crystal morphology to tune the optics of these systems. By following guanine crystal formation in developing spiders, a crystallization mechanism is elucidated. Guanine crystallization is a “non‐classical,” multistep process involving a progressive ordering of states. Crystallization begins with nucleation of partially ordered nanogranules from a disordered precursor phase. Growth proceeds by orientated attachment of the nanogranules into platelets which coalesce into single crystals, via progressive relaxation of structural defects. Despite their prismatic morphology, the platelet texture is retained in the final crystals, which are composites of crystal lamellae and interlamellar sheets. Interactions between the macromolecular sheets and the planar face of guanine appear to direct nucleation, favoring platelet formation. These findings provide insights on how organisms control the morphology and optical properties of molecular crystals.
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