In vitro crystallization of single crystalline guanine microplates mediated by proteins extracted from carp fish Cyprinus carpio

Anhydrous guanine (AG) crystals are widely applied in organisms due to their excellent optical properties. However, the influence of biomolecules on the biomineralization process of guanine crystals in organisms remains unexplored. In this study, mixed proteins from the silver film of carp fish scales were extracted and were used as a model system to elucidate the effect of biomolecules on guanine crystallization for the first time. The mixed proteins were enriched with amino acids such as Asp, Glu, Leu and Lys. Elongated hexagonal microplates of α-form AG (α-AG) mixed with a small amount of β-form AG (β-AG) mainly exposing the (100) plane were obtained in the presence of the mixed proteins under mild conditions while irregular particles were formed in the absence of proteins, indicating that the extracted mixed proteins have a strong influence on the formation of AG microplates exposing the (100) plane. It is found that the proteins not only adsorb on the surface but also occluded into the AG crystals. We also find that guanine analogues have profound synergetic effects on the morphology and polymorphs of guanine in the in vitro synthetic system. The obtained α-AG exhibit superior optical properties such as high reflectivity, strong depolarization and pearlescent luster. This in vitro synthesis work shows clearly the strong influence of biomolecules for the biomineralization process of guanine crystals in organisms.