Insights into the helix‐sense inversion of poly(β‐phenethyl‐L‐aspartate) by two‐dimensional Raman correlation spectroscopy

Abstract

In this study, the transition process of the helix-sense inversion of poly(β-phenethyl-L-aspartate) was investigated by Raman scattering and 2-dimensional correlation spectroscopy. Temperature-dependent Raman spectra were obtained during the helix-sense inversion. The results of 2-dimensional correlation analysis in the spectral regions of 1600-1800 and 3200-3400 cm showed that the intensity changes of the side-chain ester C═O stretching bands occurred prior to those of amide A and amide I bands in the unwinding process of αR-helix on heating. The sequential order of the intensity changes for amide A, amide I, and the side-chain ester C═O stretching bands during the inversion process was determined. It was found that the conformation change of the side chain occurred prior to that of the main chain for the αR-helix on heating. Thus, we concluded that the transformation of the backbone chain from right-handed to left-handed is triggered by the conformational change of the side chains.

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 .…”