Formation process of the twinning β-form anhydrous guanine platelets in the scallop eyes

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