Packing of Helices: Is Chirality the Highest Crystallographic Symmetry?

Abstract

Chiral structures resulting from the packing of helices are common in biological and synthetic materials. Herein, we analyze the noncentrosymmetry (NCS) in such systems using crystallographic considerations. A comparison of the chiral structures built from helices shows that the chirality can be expected for specific building units such as 31/32 or 61/65 helices which, in hexagonal arrangement, will more likely lead to a chiral resolution. In these two systems, we show that the highest crystallographic symmetry (i.e., the symmetry which can describe the crystal structure from the smallest assymetric unit) is chiral. As an illustration, we present the synthesis of two materials ([Zn(2,2’-bpy)3](NbF6)2 and [Zn(2,2’-bpy)3](TaF6)2) in which the 3n helices pack into a chiral structure.

Keywords

• Crystallography
• Chirality
• Helical System
• Oxide-fluorides