Department of Physics, Indian Institute of Technology Ropar, Rupnagar, Punjab 140001, India.
Department of Physics and Mathematics, School of Science and Technology, Nottingham Trent University, Clifton Campus, Nottingham NG11 8NS, U.K.
J Am Chem Soc. 2023 May 3;145(17):9646-9654. doi: 10.1021/jacs.3c00701. Epub 2023 Apr 24.
The specificity of ions in inducing conformational changes in macromolecules is introduced as the Hofmeister series; however, the detailed underlying mechanism is not comprehensible yet. We utilized surface-specific sum frequency generation (SFG) vibrational spectroscopy to explore the Hofmeister effect at the air/polyvinylpyrrolidone (PVP)/water interface. The spectral signature observed from the ssp polarization scheme reveals ion-specific ordering of water molecules following the Hofmeister series attributed to the ion-macromolecule interactions. Along with this, the presence of ions does not reflect any significant influence on the structure of the PVP macromolecule. However, the ppp-SFG spectra in the CH-stretch region reveal the impact of ions on the orientation angle of vinyl chain CH-groups, which follows the Hofmeister series: SO > Cl > NO > Br > ClO > SCN. The minimal orientation angle of CH-groups indicates significant reordering in PVP vinyl chains in the presence of chaotropic anions ClO, and SCN. The observation is attributed to the ion-specific water-macromolecule interactions at the air/aqueous interface. It is compelling to observe the signature of spectral blue shifts in the OH-stretch region in the ppp configuration in the presence of chaotropic anions. The origin of spectral blue shifts has been ascribed to the existence of weaker interactions between the interfacial water molecules and the backbone CH- and CH-moieties of the PVP macromolecules. The ion-specific modulation in water-macromolecule interactions is endorsed by the relative propensity of anion's adsorption toward the air/aqueous interface. The experimental findings highlight the existence and cooperative participation of ion-specific water-macromolecule interactions in the mechanism of the Hofmeister effect, along with the illustrious ion-water and ion-macromolecule interactions.
我们利用表面特定的和频产生(SFG)振动光谱来研究空气/聚乙烯吡咯烷酮(PVP)/水界面上的霍夫迈斯特效应。从 ssp 偏振方案观察到的光谱特征揭示了水分子按照霍夫迈斯特序列的离子特异性有序排列,这归因于离子-大分子相互作用。与此同时,离子的存在并不反映对 PVP 大分子结构的任何显著影响。然而,CH 伸缩区域的 ppp-SFG 光谱揭示了离子对乙烯基链 CH 基团取向角度的影响,这遵循霍夫迈斯特序列:SO > Cl > NO > Br > ClO > SCN。CH 基团的最小取向角度表明,在离液盐 ClO 和 SCN 的存在下,PVP 乙烯基链发生了显著的重排。这种观察归因于空气/水界面上离子特异性的水-大分子相互作用。在存在离液盐的情况下,在 ppp 配置中观察到 OH 伸缩区域光谱蓝移的特征是引人注目的。光谱蓝移的起源归因于界面水分子与 PVP 大分子的骨架 CH 和 CH 部分之间存在较弱的相互作用。离子特异性调制水-大分子相互作用得到了阴离子向空气/水界面吸附的相对倾向的支持。实验结果强调了离子特异性水-大分子相互作用在霍夫迈斯特效应机制中的存在和协同参与,以及离子-水和离子-大分子相互作用的重要性。