Department of Chemistry, Korea University, Seoul 136-701, South Korea.
J Chem Phys. 2012 Mar 28;136(12):124501. doi: 10.1063/1.3694036.
Specific ion effects on water dynamics and local solvation structure around a peptide are important in understanding the Hofmeister series of ions and their effects on protein stability in aqueous solution. Water dynamics is essentially governed by local hydrogen-bonding interactions with surrounding water molecules producing hydration electric field on each water molecule. Here, we show that the hydration electric field on the OD bond of HOD molecule in water can be directly estimated by measuring its OD stretch infrared (IR) radiation frequency shift upon increasing ion concentration. For a variety of electrolyte solutions containing Hofmeister anions, we measured the OD stretch IR bands and estimated the hydration electric field on the OD bond to be about a hundred MV∕cm with standard deviation of tens of MV∕cm. As anion concentration increases from 1 to 6 M, the hydration electric field on the OD bond decreases by about 10%, indicating that the local H-bond network is partially broken by dissolved ions. However, the measured hydration electric fields on the OD bond and its fluctuation amplitudes for varying anions are rather independent on whether the anion is a kosmotrope or a chaotrope. To further examine the Hofmeister effects on H-bond solvation structure around a peptide bond, we examined the amide I' and II' mode frequencies of N-methylacetamide in various electrolyte D(2)O solutions. It is found that the two amide vibrational frequencies are not affected by ions, indicating that the H-bond solvation structure in the vicinity of a peptide remains the same irrespective of the concentration and character of ions. The present experimental results suggest that the Hofmeister anionic effects are not caused by direct electrostatic interactions of ions with peptide bond or water molecules in its first solvation shell. Furthermore, even though the H-bond network of water is affected by ions, thus induced change of local hydration electric field on the OD bond of HOD is not in good correlation with the well-known Hofmeister series. We anticipate that the present experimental results provide an important clue about the Hofmeister effect on protein structure and present a discussion on possible alternative mechanisms.
特定离子对水动力学和多肽周围局部溶剂化结构的影响对于理解离子的豪夫迈斯特序列及其对蛋白质在水溶液中稳定性的影响非常重要。水动力学主要由与周围水分子的局部氢键相互作用决定，从而在每个水分子上产生水合电场。在这里，我们表明可以通过测量水中 HOD 分子 OD 键的 OD 伸缩红外（IR）辐射频率随离子浓度增加的偏移来直接估计 HOD 分子 OD 键上的水合电场。对于含有豪夫迈斯特阴离子的各种电解质溶液，我们测量了 OD 伸缩 IR 带，并估计 OD 键上的水合电场约为一百毫伏/厘米，标准偏差为几十毫伏/厘米。随着阴离子浓度从 1 增加到 6 M，OD 键上水合电场降低约 10%，表明溶解离子部分破坏了局部氢键网络。然而，对于不同的阴离子，测量的 OD 键上的水合电场及其波动幅度与其是亲液盐还是憎液盐无关。为了进一步研究豪夫迈斯特效应对肽键周围氢键溶剂化结构的影响，我们在各种电解质 D2O 溶液中检查了 N-甲基乙酰胺的酰胺 I'和 II'模式频率。结果发现，两种酰胺振动频率不受离子影响，这表明肽键附近的氢键溶剂化结构保持不变，与离子的浓度和性质无关。目前的实验结果表明，豪夫迈斯特阴离子效应不是由离子与肽键或其第一溶剂化壳层中的水分子的直接静电相互作用引起的。此外，尽管离子会影响氢键网络，但因此诱导的 HOD 中 OD 键上局部水合电场的变化与著名的豪夫迈斯特序列没有很好的相关性。我们预计，目前的实验结果为豪夫迈斯特对蛋白质结构的影响提供了一个重要线索，并对可能的替代机制进行了讨论。
