Institute of High Performance Computing, 1 Fusionopolis Way, #16-16, Connexis, Singapore 138632, Republic of Singapore.
Phys Chem Chem Phys. 2011 Nov 7;13(41):18647-60. doi: 10.1039/c1cp22056j. Epub 2011 Sep 22.
The influence of eight different ionic liquid (IL) solvents on the stability of the lipase Candida antarctica lipase B (CAL-B) is investigated with molecular dynamics (MD) simulations. Considered ILs contain cations that are based either on imidazolium or guanidinium as well as nitrate, tetrafluoroborate or hexafluorophosphate anions. Partial unfolding of CAL-B is observed during high-temperature MD simulations and related changes of CAL-B regarding its radius of gyration, surface area, secondary structure, amount of solvent close to the backbone and interaction strength with the ILs are evaluated. CAL-B stability is influenced primarily by anions in the order NO(3)(-)≪ BF(4)(-) < PF(6)(-) of increasing stability, which agrees with experiments. Cations influence protein stability less than anions but still substantially. Long decyl side chains, polar methoxy groups and guanidinium-based cations destabilize CAL-B more than short methyl groups, other non-polar groups and imidazolium-based cations, respectively. Two distinct causes for CAL-B destabilization are identified: a destabilization of the protein surface is facilitated mostly by strong Coulomb interactions of CAL-B with anions that exhibit a localized charge and strong polarization as well as with polar cation groups. Surface instability is characterized by an unraveling of α-helices and an increase of surface area, radius of gyration and protein-IL total interaction strength of CAL-B, all of which describe a destabilization of the folded protein state. On the other hand, a destabilization of the protein core is facilitated when direct core-IL interactions are feasible. This is the case when long alkyl chains are involved or when particularly hydrophobic ILs induce major conformational changes that enable ILs direct access to the protein core. This core instability is characterized by a disintegration of β-sheets, diffusion of ions into CAL-B and increasing protein-IL van der Waals interactions. This process describes a stabilization of the unfolded protein state. Both of these processes reduce the folding free energy and thus destabilize CAL-B. The results of this work clarify the impact of ions on CAL-B stabilization. An extrapolation of the observed trends enables proposing novel ILs in which protein stability could be enhanced further.
采用分子动力学（MD）模拟研究了八种不同离子液体（IL）溶剂对南极假丝酵母脂肪酶 B（CAL-B）稳定性的影响。所考虑的 IL 包含基于咪唑鎓或胍鎓的阳离子以及硝酸盐、四氟硼酸盐或六氟磷酸盐阴离子。在高温 MD 模拟过程中观察到 CAL-B 的部分展开，并评估了 CAL-B 与它的回转半径、表面积、二级结构、靠近骨架的溶剂数量以及与 IL 相互作用强度有关的相关变化。CAL-B 的稳定性主要受阴离子影响，稳定性顺序为 NO(3)(-)≪ BF(4)(-) < PF(6)(-)，这与实验结果一致。阳离子对蛋白质稳定性的影响小于阴离子，但仍然很大。长的癸基侧链、极性甲氧基基团和胍鎓基阳离子比短的甲基基团、其他非极性基团和咪唑鎓基阳离子更能使 CAL-B 不稳定。确定了导致 CAL-B 失稳的两个不同原因：CAL-B 与表现出局部电荷和强极化以及与极性阳离子基团的阴离子的强库仑相互作用，促进了蛋白质表面的失稳。表面不稳定性的特征是α-螺旋的解开以及表面积、回转半径和 CAL-B 的蛋白质-IL 总相互作用强度的增加，所有这些都描述了折叠蛋白质状态的失稳。另一方面，当直接的核心-IL 相互作用可行时，蛋白质核心的失稳会得到促进。当涉及长烷基链或特别疏水的 IL 诱导主要构象变化以使 IL 能够直接进入蛋白质核心时，就是这种情况。这种核心不稳定性的特征是β-折叠的解体、离子扩散到 CAL-B 中以及增加蛋白质-IL 范德华相互作用。这个过程描述了未折叠蛋白质状态的稳定。这两个过程都降低了折叠自由能，从而使 CAL-B 失稳。这项工作的结果阐明了离子对 CAL-B 稳定性的影响。对观察到的趋势进行外推，使我们能够提出进一步增强蛋白质稳定性的新型 IL。
