Monhemi Hassan, Housaindokht Mohammad Reza, Moosavi-Movahedi Ali Akbar, Bozorgmehr Mohammad Reza
Biophysical chemistry laboratory, Department of Chemistry, Faculty of Science, Ferdowsi University of Mashhad, Mashhad, Iran.
Phys Chem Chem Phys. 2014 Jul 28;16(28):14882-93. doi: 10.1039/c4cp00503a.
Deep eutectic solvents (DESs) are utilized as green and inexpensive alternatives to classical ionic liquids. It has been known that some of DESs can be used as solvent in the enzymatic reactions to obtain very green chemical processes. DESs are quite poorly understood at the molecular level. Moreover, we do not know much about the enzyme microstructure in such systems. For example, how some hydrolase can remain active and stable in a deep eutectic solvent including 9 M of urea? In this study, the molecular dynamics of DESs as a liquid was simulated at the molecular level. Urea : choline chloride as a well-known eutectic mixture was chosen as a model DES. The behavior of the lipase as a biocatalyst was studied in this system. For comparison, the enzyme structure was also simulated in 8M urea. The thermal stability of the enzyme was also evaluated in DESs, water, and 8M urea. The enzyme showed very good conformational stability in the urea : choline chloride mixture with about 66% urea (9 M) even at high temperatures. The results are in good agreement with recent experimental observations. In contrast, complete enzyme denaturation occurred in 8M urea with only 12% urea in water. It was found that urea molecules denature the enzyme by interrupting the intra-chain hydrogen bonds in a "direct denaturation mechanism". However, in a urea : choline chloride deep eutectic solvent, as a result of hydrogen bonding with choline and chloride ions, urea molecules have a low diffusion coefficient and cannot reach the protein domains. Interestingly, urea, choline, and chloride ions form hydrogen bonds with the surface residues of the enzyme which, instead of lipase denaturation, leads to greater enzyme stability. To the best of our knowledge, this is the first study in which the microstructural properties of a macromolecule are examined in a deep eutectic solvent.
深共熔溶剂（DESs）被用作传统离子液体的绿色且廉价的替代品。已知一些深共熔溶剂可在酶促反应中用作溶剂，以实现非常绿色的化学过程。在分子水平上，人们对深共熔溶剂的了解还非常有限。此外，我们对这类体系中的酶微观结构也知之甚少。例如，某些水解酶如何能在含有9M尿素的深共熔溶剂中保持活性和稳定性？在本研究中，在分子水平上模拟了作为液体的深共熔溶剂的分子动力学。选择尿素：氯化胆碱这种著名的共熔混合物作为模型深共熔溶剂。在该体系中研究了作为生物催化剂的脂肪酶的行为。为作比较，还在8M尿素中模拟了酶的结构。还评估了该酶在深共熔溶剂、水和8M尿素中的热稳定性。即使在高温下，该酶在含有约66%尿素（9M）的尿素：氯化胆碱混合物中仍表现出非常好的构象稳定性。结果与最近的实验观察结果高度吻合。相比之下，在仅含12%尿素的8M尿素水溶液中，酶完全变性。研究发现，尿素分子通过“直接变性机制”中断链内氢键使酶变性。然而，在尿素：氯化胆碱深共熔溶剂中，由于与胆碱和氯离子形成氢键，尿素分子的扩散系数较低，无法到达蛋白质结构域。有趣的是，尿素、胆碱和氯离子与酶的表面残基形成氢键，这非但没有导致脂肪酶变性，反而提高了酶的稳定性。据我们所知，这是首次在深共熔溶剂中研究大分子微观结构性质的研究。
