Conformational Change of Poly(-isopropylacrylamide) during the Coil-Globule Transition Investigated by Attenuated Total Reflection/Infrared Spectroscopy and Density Functional Theory Calculation.

A conformational change in the coil-globule transition of poly(-isopropylacrylamide) (PNiPA) was investigated by Fourier transform infrared (FT-IR) spectroscopy with attenuated total reflection (ATR) method and density functional theory (DFT) calculations. ATR/IR spectra of PNiPA in an aqueous solution change dramatically in the vicinity of the coil-globule transition temperature (θ temperature). Below the θ temperature, unimodal peaks are observed at 1624 cm in the amide I region and at 1562 cm in the amide II region, respectively. Above the θ temperature, a new peak appears abruptly near 1653 cm in the amide I region and the amide II band shifts gradually to a lower frequency by 6 cm. In the amide III region, the relative intensity of a band at 1173 cm is weaker than that of a band at 1155 cm at lower temperatures, but it becomes larger during the coil-globule transition of PNiPA. DFT calculation for dimer models of PNiPA suggests that the amide I band at 1624 cm is assigned mainly to a stretching vibration of the C═O group that forms a strong hydrogen bond with the N-H bond of a neighboring amide group. The band at 1653 cm observed above the θ temperature may be due to a free C═O group. It is, therefore, suggested that some of the intramolecular hydrogen bonds between neighboring amide groups are broken during the coil-globule transition. Furthermore, it is deduced from the DFT calculation that the relative intensity of the bands at 1173 and 1155 cm in the amide III region reflects the population change in the gauche and trans conformations in the main chain during the coil-globule transition.