Structure Determination of Biogenic Crystals Directly from 3D Electron Diffraction Data

Highly reflective assemblies of purine, pteridine, and
flavin crystals
are used in the coloration and visual systems of many different animals.
However, structure determination of biogenic crystals by single-crystal
XRD is challenging due to the submicrometer size and beam sensitivity
of the crystals, and powder XRD is inhibited due to the small volumes
of powders, crystalline impurity phases, and significant preferred
orientation. Consequently, the crystal structures of many biogenic
materials remain unknown. Herein, we demonstrate that the 3D electron
diffraction (3D ED) technique provides a powerful alternative approach,
reporting the successful structure determination of biogenic guanine
crystals (from spider integument, fish scales, and scallop eyes) from
3D ED data confirmed by analysis of powder XRD data. The results show
that all biogenic guanine crystals studied are the previously known
β-polymorph. This study highlights the considerable potential
of 3D ED for elucidating the structures of biogenic molecular crystals
in the nanometer-to-micrometer size range. This opens up an important
opportunity in the development of organic biomineralization, for which
structural knowledge is critical for understanding the optical functions
of biogenic materials and their possible applications as sustainable,
biocompatible optical materials.