Different Origins of Strain-Induced Chirality Inversion of Co2+-Triggered Supramolecular Peptide Polymers

Abstract

We report a distinctly different dynamic helix inversion pathway of self-assembled terpyridine-based ligands composed of different numbers of peptide moieties with Co2+ and its amplification of strain-induced chirality from an achiral terpyridine moiety. The helical chirality of the metal centers, coordinated by terpyridine ligands, is controlled by strain-induced chirality with complex ligand-to-Co2+ ratios. We also show that the distinct helical inversion mechanism is significantly dependent on the number of peptides attached to ligands. The helical inversion pathway of the self-assembled ligand (R-1 and S-1) complexes composed of one alanine analogue (R- or S-2-amino-1-propyl moiety) and one long saturated alkyl chain relies on two steps of chirality with different complex geometries, first from strain-induced chirality originating from an octahedral structure to octahedral structure with different helical direction and then on to helical chirality in a square-pyramidal structure. In contrast, the helix inversion of the self assembled R-2 and S-2 complexes containing an alanine analogue and two glycine moieties with Co2+ was followed by one step to form two distinct coexisting complex geometries having the same helical direction. In particular, the circular dichroism (CD) intensities of the self-assembled R-1 and R-2 complexes with Co2+ were 900-1500-fold amplified compared to those of free R-1 and R-2. The Gibbs free energies of the self-assembled complexes with different geometries were also calculated by temperature dependent CD observation; the square-pyramidal structure of the self-assembled R-1 complex with Co2+ was more stable than the self-assembled R-2 complex with Co2+. Furthermore, the self-assembled R-1 and S-1 complexes with 1.0 equiv of Co2+ could classify amino acids by their chirality.