Discovering the Crystal Structure of Biogenic Xanthine Crystals

Thin xanthine crystal plates operate as mirrors in the ocelli (small eyes) of the insect L. rozsypali (jumping bristletail), where they form superstructures that are used to reflect missed incident light back to the light detectors. Here, we present the structure of the biogenic xanthine crystals. Structure determination is made possible by combining electron diffraction and 4D scanning transmission electron microscopy with fundamental symmetry considerations and first-principles calculations based on density functional theory. We find three possible structures, all layered, with individual planes comprised of the same planar hydrogen-bonded network. However, they are different polytypes (i.e., exhibit different stacking of the individual planes). One of these polytypes corresponds to the structure of the biogenic crystals, whereas a second polytype corresponds to the structure recently determined for synthetic xanthine crystals.