Sparkling Reflective Stacks of Purine Crystals in the Nudibranch Flabellina iodinea

Rationalizing the Influence of Small-Molecule Dopants on Guanine Crystal Morphology

Many spectacular optical phenomena in animals are produced by reflective assemblies of guanine crystals. The crystals comprise planar H-bonded layers of π-stacked molecules with a high in-plane refractive index. By preferentially expressing the highly reflective π-stacked (100) crystal face and controlling its cross-sectional shape, organisms generate a diverse array of photonic superstructures. How is this precise control over crystal morphology achieved? Recently, it was found that biogenic guanine crystals are composites, containing high quantities of hypoxanthine and xanthine in a molecular alloy. Here, we crystallized guanine in the presence of these dopants and used computations to rationalize their influence on the crystal morphology and energy. Exceptional quantities of hypoxanthine are incorporated into kinetically favored solid solutions, indicating that fast crystallization kinetics underlies the heterogeneous compositions of biogenic guanine crystals. We find that weakening of H-bonding interactions by additive incorporation elongates guanine crystals along the stacking direction-the opposite morphology of biogenic crystals. However, by modulation of the strength of competing in-plane H-bonding interactions, additive incorporation strongly influences the cross-sectional shape of the crystals. Our results suggest that small-molecule H-bond disrupting additives may be simultaneously employed with π-stack blocking additives to generate reflective platelet crystal morphologies exhibited by organisms.

Intermediate filaments spatially organize intracellular nanostructures to produce the bright structural blue of ribbontail stingrays across ontogeny

In animals, pigments but also nanostructures determine skin coloration, and many shades are produced by combining both mechanisms. Recently, we discovered a new mechanism for blue coloration in the ribbontail stingray Taeniura lymma, a species with electric blue spots on its yellow-brown skin. Here, we characterize finescale differences in cell composition and architecture distinguishing blue from non-blue regions, the first description of elasmobranch chromatophores and the nanostructures responsible for the stingray's novel structural blue, contrasting with other known mechanisms for making nature's rarest color. In blue regions, the upper dermis comprised a layer of chromatophore units -iridophores and melanophores entwined in compact clusters framed by collagen bundles- this structural stability perhaps the root of the skin color's robustness. Stingray iridophores were notably different from other vertebrate light-reflecting cells in having numerous fingerlike processes, which surrounded nearby melanophores like fists clenching a black stone. Iridophores contained spherical iridosomes enclosing guanine nanocrystals, suspended in a 3D quasi-order, linked by a cytoskeleton of intermediate filaments. We argue that intermediate filaments form a structural scaffold with a distinct optical role, providing the iridosome spacing critical to produce the blue color. In contrast, black-pigmented melanosomes within melanophores showed space-efficient packing, consistent with their hypothesized role as broadband-absorbers for enhancing blue color saturation. The chromatophore layer's ultrastructure was similar in juvenile and adult animals, indicating that skin color and perhaps its ecological role are likely consistent through ontogeny. In non-blue areas, iridophores were replaced by pale cells, resembling iridophores in some morphological and nanoscale features, but lacking guanine crystals, suggesting that the cell types arise from a common progenitor cell. The particular cellular associations and structural interactions we demonstrate in stingray skin suggest that pigment cells induce differentiation in the progenitor cells of iridophores, and that some features driving color production may be shared with bony fishes, although the lineages diverged hundreds of millions of years ago and the iridophores themselves differ drastically.

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

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.

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

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.