Helicity-driven chiral self-sorting supramolecular polymerization with Ag⁺: right- and left-helical aggregates

Abstract

The study of chiral self-sorting is extremely important for understanding biological systems and for developing applications for the biomedical field. In this study, we attempted unprecedented chiral self-sorting supramolecular polymerization accompanying helical inversion with Ag⁺ in one enantiomeric component. Bola-type terpyridine-based ligands (<i>R</i>-L¹ and <i>S</i>-L¹) comprising <i>R</i>- or <i>S</i>-alanine analogs were synthesized. First, <i>R</i>-L¹ dissolved in DMSO/H₂O (1 : 1, v/v) forms right-handed helical fibers (aggregate I) <i>via</i> supramolecular polymerization. However, after the addition of AgNO₃ (0.2-1.1 equiv.) to the <i>R</i>-L¹ ligand, in particular, it was found that aggregate II with left-handed helicity is generated from the [<i>R</i>-L¹(AgNO₃)₂] complex through the [<i>R</i>-L¹Ag]⁺ complex <i>via</i> the dissociation of aggregate I by a multistep with an off pathway, thus demonstrating interesting self-sorting properties driven by helicity and shape discrimination. In addition, the [<i>R</i>-L¹(AgNO₃)₂] complex, which acted as a building block to generate aggregate III with a spherical structure, existed as a metastable product during the formation of aggregate II in the presence of 1.2-1.5 equiv. of AgNO₃. Furthermore, the AFM and CD results of two samples prepared using aggregates I and III with different volume ratios were similar to those obtained upon the addition of AgNO₃ to free <i>R</i>-L¹. These findings suggest that homochiral self-sorting in a mixture system occurred by the generation of aggregate II composed of the [<i>R</i>-L¹Ag]⁺ complex <i>via</i> the rearrangement of both, aggregates I and III. This is a unique example of helicity- and shape-driven chiral self-sorting supramolecular polymerization induced by Ag⁺ starting from one enantiomeric component. This research will improve understanding of homochirality in complex biological models and contribute to the development of new chiral materials and catalysts for asymmetric synthesis.