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.
Highly reflective crystals of small organic molecules are the functional materials in a wide variety of optical systems in animals. We present a perspective on this field of organic biomineralization and review evidence for the widespread distribution of organic crystals in animals. These materials were, until recently, largely overlooked principally because the crystals are lost during conventional electron microscopy preparation procedures. We document the discovery of the crystalline pteridines as a new class of biological crystals and explain why many more organic biocrystals will likely be discovered and where they may be found. We examine the chemical basis for the extraordinary optical properties of the crystals and review some of their newly discovered biological functions. The study of biogenic molecular crystals promises to yield fascinating insights into photonic systems in animals and will inspire the development of artificial optical materials.
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