Structure and Morphology of Light-Reflecting Synthetic and Biogenic Polymorphs of Isoxanthopterin: A Comparison

Until
recently it was thought that the only optical function of
pteridines in biology was to act as light-absorbing pigments, but
a recent report by some of us revealed that crystalline isoxanthopterin
is a reflector in the eyes of decapod crustaceans. Here, we report
the formation of crystalline isoxanthopterin synthetically from the
polar dimethyl sulfoxide solvent, with X-ray diffraction analysis
revealing a crystal structure different from that of biogenic isoxanthopterin.
The structure of the new polymorph was determined in two independent
ways. In one approach, it was generated and optimized using first-principles
calculations, followed by comparison of simulation and experiment
for high-resolution powder X-ray diffraction (PXRD) and electron diffraction.
In the other approach, the structure was obtained definitively from
PXRD data using a direct-space genetic algorithm for structure solution
followed by Rietveld refinement. The synthetic structure is different
from its biogenic counterpart, especially in having a nonplanar criss-cross
H-bonded arrangement. We also rationalized the morphology of the crystals
and the effect of the DMSO thereon, via a comparison between observed
and theoretical growth morphologies. In addition, we calculated the
optical properties of the synthetic structure and found its two dominant
refractive indices to be somewhat lower than those of its biogenic
counterpart, but still as high as those of reflecting guanine crystals.
Synthetic isoxanthopterin therefore emerges as a promising candidate
for incorporation in artificial optical systems.