The Formation Mechanisms of Biogenic Guanine Crystals

Many optical phenomena in animal coloration and vision are generated by interactions of light with assemblies of guanine crystals.^1,2 Guanine crystals comprise π-stacked, H-bonded layers, with a high in-plane refractive index. Organisms precisely control crystal morphology to form plate-like crystals preferentially expressing the highly reflective, hydrophobic (100) crystal face parallel to the H-bonds. While crystals grown <i>in vitro</i> form bulky prismatic morphologies that are not particularly optically useful. Our objective is to discover the strategies organisms use to exquisitely regulate crystal morphology and harness these strategies to develop new optically functional molecular materials. We approached this by following crystal formation in model organisms undergoing development using electron microscopy and <i>in situ</i> diffraction techniques. By studying hatchling white-widow spiders, we found that guanine crystals are composites and that their crystallization is a ‘non-classical’, multi-step process involving a progressive ordering of states.³ In juvenile scallop eyes, we showed that pre-assembled, fibrillar sheets provide an interface for nucleation and direct the growth and orientation of plate-like guanine crystals.⁴ In addition, we test these strategies by <i>in vitro</i> crystallization experiments, providing a rationalization for the crystal morphologies found in organisms. Thus, crystal-producing organisms can inspire new approaches for manipulating the morphologies and properties of molecular materials.<br/><br/>1. Gur, D., Palmer, B. A., Weiner, S. & Addadi, L. Light manipulation by guanine crystals in organisms: Biogenic scatterers, mirrors, multilayer reflectors and photonic crystals. <i>Adv. Funct. Mater.</i> <b>27</b>, 1603514 (2017).<br/>2. Palmer, B. A., Gur, D., Weiner, S., Addadi, L. & Oron, D. The organic crystalline materials of vision: structure–function considerations from the nanometer to the millimeter scale. <i>Adv. Mater.</i> <b>30</b>, 1800006 (2018).<br/>3. Wagner, A. <i>et al.</i> The non-classical crystallization mechanism of a composite biogenic guanine crystals. <i>Adv. Mater.</i> <b>34</b>, 2202242 (2022).<br/>4. Wagner, A. <i>et al.</i> Macromolecular sheets direct the morphology and orientation of plate-like biogenic guanine crystals. <i>Nat. Commun.</i> <b>14</b>, 589 (2023).