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

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(N-propargylamide) with an l- or d-alanine residue as the pendant (P-l(or -d)-ala-NH2) can be induced to form a predominantly one-handed helical structure in an aqueous solution by complexing with metal ions, including Cu2+ 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 Cu2+ was used as an inducer, the CD and UV–vis absorptions of P-l-ala-NH2 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 Cu2+ 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.