Macromolecular Chiral Amplification through a Random Coil to One-Handed Helix Transformation Induced by Metal Ion Coordination in an Aqueous Solution

Abstract

Many inspiring achievements have been made in inducing chiral amplification in helical polymers; however, the induction of helical chirality in random coil polymers by the intermolecular noncovalent force in solutions so far is still an intractable task, which is almost only accomplishable in biopolymer systems. In this contribution, we found that a random coil poly(Npropargylamide) with an L-or D-alanine residue as the pendant (P-L(or -D)-ala-NH 2 ) can be induced to form a predominantly onehanded helical structure in an aqueous solution by complexing with metal ions, including Cu 2+ and Ag + . The random coil to predominantly one-handed helix transformation and the chiral amplification therein were clearly evidenced by the remarkable red-shift of the UV−vis absorption peak, the occurrence of strong circular dichroism (CD), and the sharp increase in the absolute value of specific rotation. When Cu 2+ was used as an inducer, the CD and UV−vis absorptions of P-L-ala-NH 2 were barely affected by heating and changing anions, demonstrating the high stability of the induced helical structure. The results obtained from CD, UV−vis, resonance Raman, Fourier transform infrared (FT-IR), and X-ray photoelectron spectroscopies and differential scanning calorimetry indicate that a contracted cis-cisoidal helical structure was formed in the polymer main chain through the generation of cis nearly square-planar complexes between each Cu 2+ ion and the four nitrogens of two pendants. The metal ion coordination concurrently provided appropriate linkage among the pendants and reduced the pendant flexibility, thus endowing the main chain with helicity and triggering chiral amplification. Inspired by this mechanism, a universal and convenient methodology for preparing helical polymer/metal ion complexes and the corresponding chiral materials from random coil polymers may be established.

Metal-Enhanced Helical Chirality of Coil Macromolecules: Bioinspired by Metal Coordination-Induced Protein Folding

“…However, the addition of 25% of the isomer considerably changed the specific rotation of poly-( l 0.75 - co - d 0.25 )-ala-Boc or poly-( l 0.25 - co - d 0.75 )-ala-Boc. As reported in our previous study, poly- l -ala-Boc and poly- d -ala-Boc realized a helical conformation in the CHCl 3 solution . Interestingly, poly-( l 0.9 - co - d 0.1 )-ala-Boc and poly-( l 0.1 - co - d 0.9 )-ala-Boc exhibited almost the same CD spectral patterns as homopolymers (Figure b), indicating that the one-handed helical structure of poly-( l 0.9 - co - d 0.1 )-ala-Boc or poly-( l 0.1 - co - d 0.9 )-ala-Boc hardly changes.…”

“…In previous studies, 50 the P-L(or-D)-ala-NH 2 /Cu 2+ complex in the NIR region (450−850 nm) exhibited clear CD (Figure 4b), which belonged to different orbital d−d* transitions due to the d orbital energy level splitting of Cu 2+ , 58 where a nearly square structure was formed by coordinating Cu 2+ with four nitrogen atoms of two amino-acid amides (Figure 3d). This result further demonstrated that a special interaction between the poly-(L-co-D)-ala-NH 2 and Cu 2+ was metal coordination.…”

Tunable induced circular dichroism in gels

“…Chiral assemblies with controllable ICDs open up new perspectives in the field of chiroptical applications, including but not limited to optical amplifier materials, 23,36,37 chiral sensing, 38,39 optical switch, and data storage applications. [40][41][42] Furthermore, a research group in Spain has shown that the supramolecular gels can be applied as organic reaction media.…”

Thermoswitching of Helical Inversion of Dynamic Polyphenylacetylenes through cis-trans Isomerization of Amide Pendants

“…However, most notably, the conformation adjustment of subunit structures can feasibly induce helical inversion. Tunable helical senses of polymers by changing the polarity, donor character, solvation ability of the solvent, or complexation with metal ions have been described. − For example, Yashima et al reported that the screw senses of polyphenylacetylene (PPA) bearing achiral phosphonic acid monoesters as the pendant groups were inverted by the adjustment of the asymmetrical position in phosphorus pendants or chiral amine additives in different solvents . Riguera and Freire et al demonstrated that the sp/ap equilibrium in (O)­C–C­(−O) at the pendants was shifted to change the helical senses by complexation with different metal ions. − Interestingly, their work involves the cis-trans isomerization of secondary amides induced by changing the donor character of solvents, which would modulate both the elongation and rotation of PPA .…”