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 biogenic xanthine crystals and compare it to that determined for synthetic xanthine crystals. Structure determination is made possible by combining electron diffraction and x-ray diffraction, as well as 4D scanning transmission electron microscopy, with fundamental symmetry considerations and first principles calculations based on density functional theory. We find that both biogenic and synthetic structures are layered, with individual planes comprised of a planar hydrogen-bonded network. However, they are different polytypes (i.e., exhibit different stacking of the individual planes).