School of Physical and Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link, Singapore.
J Phys Chem B. 2012 Feb 2;116(4):1446-51. doi: 10.1021/jp210769q. Epub 2012 Jan 20.
The mechanism of urea-induced protein denaturation is explored through studying the salting effect of urea on 14 amino acid side chain analogues, and N-methylacetamide (NMA) which mimics the protein backbone. The solvation free energies of the 15 molecules were calculated in pure water, aqueous urea, and NaCl solutions. Our results show that NaCl displays strong capability to salt out all 15 molecules, while urea facilitates the solvation (salting-in) of all the 15 molecules on the other hand. The salting effect is found to be largely enthalpy-driven for both NaCl and urea. Our observations can explain the higher stability of protein's secondary and tertiary structures in typical salt solutions than that in pure water. Meanwhile, urea's capability to better solvate protein backbone and side-chain components can be extrapolated to explain protein's denaturation in aqueous urea solution. Urea salts in molecules through direct binding to solute surface, and the strength is linearly dependent on the number of heavy atoms of solute molecules. The van der Waals interactions are found to be the dominant force, which challenges a hydrogen-bonding-driven mechanism proposed previously.
通过研究尿素对 14 种氨基酸侧链类似物和模拟蛋白质主链的 N-甲基乙酰胺（NMA）的盐效应，探索了尿素诱导蛋白质变性的机制。在纯水中、尿素水溶液和 NaCl 溶液中计算了这 15 个分子的溶剂化自由能。结果表明，NaCl 具有很强的盐析能力，可以将所有 15 个分子盐析出来，而另一方面，尿素促进了所有 15 个分子的溶解（盐溶）。对于 NaCl 和尿素，盐效应主要是由焓驱动的。我们的观察结果可以解释为什么蛋白质的二级和三级结构在典型盐溶液中的稳定性比在纯水中更高。同时，尿素能够更好地溶解蛋白质的主链和侧链成分，可以推断出它在水合尿素溶液中使蛋白质变性的能力。尿素通过直接与溶质表面结合形成盐，其强度与溶质分子的重原子数呈线性关系。研究发现，范德华相互作用是主要作用力，这挑战了先前提出的氢键驱动机制。
