Formation mechanism of twinned β-form anhydrous guanine platelets in scallop eyes

Square shaped twinned guanine microplatelets with high symmetry are assembled into highly ordered layered patterns and function as image-forming mirrors in scallop eyes. However, the twinning mechanism of twinned guanine microplatelets is still unclear. Herein, the eyes of juvenile Yesso scallops were investigated to understand the formation mechanism of the twinned β-form anhydrous guanine (β-AG) microplatelets exposing the (100) plane. We found α form anhydrous guanine (α-AG) and single-crystal β-AG nanoplatelets in the very early stage of the eyes of the juvenile scallops, while the α-AG was supposed to be formed via amorphous guanine during the sample preparation process. Besides β-AG and α-AG, amorphous guanine was found in the eyes of juvenile scallops with a size of 2.5 mm according to the Raman spectra. A formation mechanism was proposed for the biogenic twinned β-AG platelets. Firstly, amorphous guanine is formed as an intermediate phase, which transforms into single crystalline β-AG nanoplatelets, or dissolves and recrystallizes to single crystalline β-AG nanoplatelets. Then, a second layer of β-AG forms on the top of the original single crystalline β-AG nanoplatelets, forming twinned β-AG nanoplatelets with two c axes with a certain angle, 83° or 14°. Each layer of the β-AG nanoplatelets is calculated to be about 14 ± 2 nm. This is the first time the twinning mechanism of biogenic twinned β-AG microplatelets is reported. Uncovering the formation mechanism of twinned platelets of organic crystals may shed light on the formation of functional synthetic twinned organic crystals in laboratories.