Fig. 6. Mechanism leading to chiral switching as induced by chiral amino acid enantiomer.

(A) SEM image of a nearly achiral vaterite helicoid showing the clockwise rotation of 22.5° between two connected vertical platelets in adjacent mother (blue) and daughter (purple) layers in the rotated-platelet region. (B) A model at the (100) plane for two adjacent vertical and rotated vaterite platelet layers showing details of a consequential rotated daughter vertical platelet position relative to the mother vertical platelet position, with intervening amino acid layer, as identified by RosettaSurface computational simulation fixing the clockwise rotation at 22.5°. Vaterite crystal atoms: Ca, green; C, gray in mother layer and yellow in daughter layer; O, red. (C) High-resolution simplified image showing the configuration at the 22.5°rotation between the bottom mother vaterite layer with exposed surface calcium (green), the intervening middle l-Asp layer (blue), and the top daughter vaterite layer with exposed surface carbonates (yellow). (D) Schematic summary (of a top view) of the chiral switching of surface structures in vaterite helicoids caused by the 22.5° layer-by-layer clockwise rotation in the presence of the l-enantiomer of Asp, whose replication/amplification with further growth leads to the 180° chiral switch cycle. The entire cycle is composed of four steps with eight continuous 22.5° clockwise-rotated layers: (i) an achiral straight-radiating vertical platelet start point at 0°; (ii) a first clockwise platelet rotational step of 22.5°, which repeats in subsequent growth layers to reach 45°, and then 67.5° from the start point resulting in a clockwise chiral helicoid; (iii) a transitional horizontal-surrounding platelet orientation step that occurs at 90° from the start point, resulting in an achiral helicoid; and (iv) continued 22.5° clockwise platelet rotations to reach 112.5°, then 135°, and then 157.5° from the start point, resulting in a counterclockwise chiral helicoid. (E and F) Two-dimensional (2D) (top view) and 3D (side view) schematics of the relationship at one location for one chiral switching cycle of eight successional clockwise rotations of 22.5° of vertical platelet layer configurations, and the chiral status of the corresponding whole vaterite helicoids, as produced in the presence of the l-enantiomer of Asp.