Katsumoto Yukiteru, Tanaka Takeyuki, Sato Harumi, Ozaki Yukihiro
Department of Chemistry, School of Science, Kwansei-Gakuin University, 2-1 Gakuen, Sanda, Hyogo 669-1337, Japan, and Graduate School of Science and Technology, Kobe University, Rokkodai, Nada-ku, Kobe 657-8501, Japan.
J Phys Chem A. 2002 Apr 11;106(14):3429-3435. doi: 10.1021/jp0124903.
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.
采用衰减全反射傅里叶变换红外光谱（ATR/FT-IR）法和密度泛函理论（DFT）计算，研究了聚（N-异丙基丙烯酰胺）（PNiPA）在卷曲-球状转变过程中的构象变化。PNiPA水溶液的ATR/IR光谱在卷曲-球状转变温度（θ温度）附近发生显著变化。在θ温度以下，酰胺I区域在1624 cm处和酰胺II区域在1562 cm处观察到单峰。在θ温度以上，酰胺I区域在1653 cm附近突然出现一个新峰，酰胺II带逐渐向低频移动6 cm。在酰胺III区域，1173 cm处谱带的相对强度在较低温度下比1155 cm处谱带的相对强度弱，但在PNiPA的卷曲-球状转变过程中其相对强度增大。PNiPA二聚体模型的DFT计算表明，1624 cm处的酰胺I带主要归属于与相邻酰胺基团的N-H键形成强氢键的C═O基团的伸缩振动。在θ温度以上观察到的1653 cm处的谱带可能归因于游离的C═O基团。因此，表明在卷曲-球状转变过程中相邻酰胺基团之间的一些分子内氢键被破坏。此外，从DFT计算推断，酰胺III区域中1173 cm和1155 cm处谱带的相对强度反映了卷曲-球状转变过程中主链中gauche和trans构象的数量变化。
