Conformational change of proline residues observed in swollen and shrunken phases: ?-Ray synthesis and variable temperature circular dichroism spectra of a thermo-responding polymer, poly(acryloyl-Pro-OMe)

Abstract

Poly(acryloyl-L-proline-methyl ester) (1) has optically active side chain, and constitutes thermoresponsive hydrogels upon crosslinking. In this study, we have prepared uncrosslinked polymer of 1 with a 10-kGy irradiation dose of γ-ray. For this polymer, 1, variable temperature circular dichroism (CD) and 1 H NMR spectra have been studied in the range of 0-30 °C. The intense CD spectrum at 0 °C suggests that the side chains in 1 have an ordered orientation. The CD intensity decreases gradually with increasing temperature. The decreased intensity of CD spectra indicates that the disordering occurs for the side-chain orientation. The CD band shape changes discontinuously at 20 °C. In the 1 H NMR spectra, signals disappear above 20 °C. These spectral change at 20 °C indicate that the phase transition occurs at around 20 °C from swollen to shrunken phase. Even after the phase transition, the CD spectra are still changing with isochromic point at 212 nm. It appears that the side-chain conformation is still changing from one state to the other state in the shrunken phase polymers.