Ertekin Umay Eren, Okur Halil Ibrahim
Department of Chemistry, Faculty of Science, Bilkent University, 06800 Ankara, Turkey.
National Nanotechnology Research Center (UNAM), Bilkent University, 06800 Ankara, Turkey.
J Phys Chem Lett. 2024 Jun 13;15(23):6151-6157. doi: 10.1021/acs.jpclett.4c00925. Epub 2024 Jun 4.
Ions influence the solution properties of macromolecules. Although much is known about anions, cationic effects are considered mostly in terms of weak interactions or exclusion from neutral interfaces. Herein, we have systematically studied the effect of quaternary tetraalkylammonium cations (NH, NMe, NEt, NPr, NBu) on the phase transition of poly(-isopropylacrylamide) (PNIPAM) in aqueous solution. Solubility measurements were coupled to H NMR and ATR-FTIR spectroscopic measurements. The solubility and NMR measurements revealed a direct binding between the greasiest cations and the isopropyl group of the macromolecule, evidenced from the nonlinear, Langmuir-type chemical shift response only at the isopropyl NMR signals with increasing salt concentrations. The ATR-FTIR measurements focusing on the amide oxygen showed that it is not the main direct-binding site. Additionally, the salting-out effects of the greasier cations correlate with their hydration entropies. These results demonstrate that the most weakly hydrated cations can bind to macromolecules as strongly as the weakly hydrated Hofmeister anions.
离子会影响大分子的溶液性质。尽管人们对阴离子已经了解很多，但阳离子效应大多是从弱相互作用或与中性界面的排斥方面来考虑的。在此，我们系统地研究了季铵型四烷基铵阳离子（NH、NMe、NEt、NPr、NBu）对水溶液中聚（N-异丙基丙烯酰胺）（PNIPAM）相变的影响。溶解度测量与1H NMR和ATR-FTIR光谱测量相结合。溶解度和NMR测量表明，最油腻的阳离子与大分子的异丙基之间存在直接结合，这从随着盐浓度增加仅异丙基NMR信号出现非线性、朗缪尔型化学位移响应得以证明。聚焦于酰胺氧的ATR-FTIR测量表明它不是主要的直接结合位点。此外，更油腻的阳离子的盐析效应与其水化熵相关。这些结果表明，水化程度最弱的阳离子能够与水化程度较弱的霍夫迈斯特阴离子一样强烈地结合到大分子上。
