Fujii Yuki, Ioka Hikaru, Minamoto Chihiro, Kurisaki Ikuo, Tanaka Shigenori, Ohta Kaoru, Tominaga Keisuke
Department of Chemistry, Graduate School of Science, Kobe University, Rokkodai-cho 1-1, Nada, Kobe 657-8501, Japan.
Department of Applied Chemistry and Biotechnology, Niihama National College of Technology, Yakumo-cho 7-1, Niihama, Ehime 792-8580, Japan.
J Chem Phys. 2024 Aug 14;161(6). doi: 10.1063/5.0218180.
Poly(N,N-diethylacrylamide) (PdEA), one of the thermoresponsive polymers, in aqueous solutions has attracted much attention because of its characteristic properties, such as coil-to-globule (CG) transition. We performed two-dimensional infrared spectroscopy and molecular dynamics (MD) simulations to understand the hydration dynamics in the vicinity of the CG transition at the molecular level via vibrational frequency fluctuations of the carbonyl stretching modes in the side chains of PdEA. Furthermore, N,N-diethylpropionamide, a repeating monomer unit of PdEA, is also investigated for comparison. From decays of the frequency-frequency time correlation functions (FFTCFs) of the carbonyl stretching modes, we consider that inhomogeneity of the hydration environments originates from various backbone configurations of PdEA. The degree of the inhomogeneity depends on temperature. Hydration water molecules near the carbonyl groups are influenced by the confinements of the polymers. The restricted reorientation of the embedded water, the local torsions of the backbone, and the rearrangement of the whole structure contribute to the slow spectral diffusion. By performing MD simulations, we calculated the FFTCFs and dynamical quantities, such as fluctuations of the dihedral angles of the backbone and the orientation of the hydration water molecules. The simulated FFTCFs match well with the experimental results, indicating that the retarded water reorientations via the excluded volume effect play an important role in the vibrational frequency fluctuations of the carbonyl stretching mode. It is also found the embedded water molecules are influenced by the local torsions of the backbone structure within the time scales of the spectral diffusion.
聚（N，N - 二乙基丙烯酰胺）（PdEA）是一种温敏聚合物，因其具有诸如线圈到球状体（CG）转变等特性，在水溶液中备受关注。我们进行了二维红外光谱和分子动力学（MD）模拟，以通过PdEA侧链中羰基伸缩模式的振动频率波动，在分子水平上理解CG转变附近的水化动力学。此外，还研究了PdEA的重复单体单元N，N - 二乙基丙酰胺以作比较。从羰基伸缩模式的频率 - 频率时间相关函数（FFTCFs）的衰减情况来看，我们认为水化环境的不均匀性源于PdEA的各种主链构型。不均匀程度取决于温度。羰基附近的水化水分子受聚合物限制的影响。嵌入水的受限重新取向、主链的局部扭转以及整个结构的重排导致了缓慢的光谱扩散。通过进行MD模拟，我们计算了FFTCFs和动力学量，例如主链二面角的波动和水化水分子的取向。模拟的FFTCFs与实验结果吻合良好，表明通过排除体积效应导致的水的延迟重新取向在羰基伸缩模式的振动频率波动中起重要作用。还发现嵌入的水分子在光谱扩散的时间尺度内受主链结构局部扭转的影响。
