Reversible helix–helix transition of poly( β-phenylbutyl l-aspartate) and poly( β-phenylpentyl l-aspartate), involving a screw-sense inversion in the solid state

Abstract

We newly synthesized poly( β-phenylbutyl l-aspartate) with the side chain R=–(CH 2) 4Ph and poly( β-phenylpentyl l-aspartate) with R=–(CH 2) 5Ph to extend the study on a helix–helix transition in the solid state. In these two polymers, the helix–helix transition takes place reversibly during a heating and cooling cycle similarly as in poly( β-phenylpropyl l-aspartate) with R=–(CH 2) 3Ph. From the combined observations by circular dichroism, X-ray, 1 3C-NMR and IR measurements, we postulated that the reversible transition is due to an interconversion of the helical screw sense between a right-handed α-helix and a left-handed ω-helix. The effect of the side-chain length on the transition behavior was examined.