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

Abstract

The cis-trans isomerization of amides plays a vital role in all kinds of structural changes and biological activities, involving protein folding, protein misfolding, and even protein molecular recognition. However, the helical inversion of synthetic macromolecules arising from the cis-trans isomerization of tertiary amides, as a good mimic of natural protein, has been rarely researched. Herein, we synthesized meta-disubstituted polyphenylacetylenes (PPAs) with different N-substituted tertiary amides as one of the pendants, poly{(−)-3-methoxycarbonyl-5-[N-methyl-N-(S)-(1-phenylethyl)carbamoyl]phenylacetylene}(sP-NMe) and poly{(−)-3-methoxycarbonyl-5-[N-propyl -N-(S)-(1-phenylethyl)carbamoyl]phenylacetylene} (sP-NPr). The cis-trans isomerization of amides was investigated by a combinational analysis of 1H/13C nuclear magnetic resonance (NMR), heteronuclear multiple quantum coherence (HMQC), variable-temperature NMR, Raman spectroscopy, ultraviolet–visible (UV–vis) spectroscopy, electronic circular dichroism (ECD), vibrational circular dichroism (VCD), and computer calculations. The substituted amide of sP-NMe favors the trans isomer at low temperatures, and the polyene main chain takes the right-handed helical conformation. When increasing the temperature, the ratio of the cis isomer in sP-NMe increases, which causes helical inversion. The screw sense of the polymer backbone of sP-NMe can be thermoreversibly switched upon tuning the cis-trans isomerization equilibrium of the tertiary amide pendant. In comparison, sP-NPr has the left-handed conformation with the majority of the cis isomer owing to the bulky propyl group, and cis-trans isomerization cannot induce helical inversion upon thermal treatment. The results reveal that helical inversion caused by the cis-trans isomerization of amide groups is highly dependent on the size of N-alkyl substituents.