Inhomogeneous distribution of constituent molecules in a mixed solvent has been known to give remarkable effects on the solute, e.g., conformational changes of biomolecules in an alcohol–water mixture. We investigated the general effects of 2,2,2-trifluoroethanol (TFE) on proteins/peptides in a mixture of water and TFE using melittin as a model protein. Fluctuations and Kirkwood–Buff integrals (KBIs) in the TFE–H₂O mixture, quantitative descriptions of inhomogeneity, were determined by small-angle X-ray scattering investigation and compared with those in the aqueous solutions of other alcohols. The concentration fluctuation for the mixtures ranks as methanol < ethanol ≪ TFE < tert-butanol < 1-propanol, indicating that the inhomogeneity of molecular distribution in the TFE–H₂O mixture is unexpectedly comparable to those in the series of mono-ols. On the basis of the concentration dependence of KBIs between the TFE molecules, it was found that a strong attraction between the TFE molecules is not necessarily important to induce helix conformation, which is inconsistent with the previously proposed mechanism. To address this issue, by combining the KBIs and the helix contents reported by the experimental spectroscopic studies, we quantitatively evaluated the change in the preferential binding parameter of TFE to melittin attributed to the coil–helix transition. As a result, we found two different regimes on TFE-induced helix formation. In the dilute concentration region of TFE below ∼2 M, where the TFE molecules are not aggregated among themselves, the excess preferential binding of TFE to the helix occurs due to the direct interaction between them, namely independent of the solvent fluctuation. In the higher concentration region above ∼2 M, in addition to the former effect, the excess preferential binding is significantly enhanced by the solvent fluctuation. This scheme should be held as general cosolvent effects of TFE on proteins/peptides.

中文翻译：

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醇诱导的螺旋-螺旋转变的两种不同机制：2,2,2-三氟乙醇对蛋白质的影响既独立于溶剂结构波动又被溶剂结构波动增强

已知混合溶剂中组成分子的不均匀分布会对溶质产生显着影响，例如，醇水混合物中生物分子的构象变化。我们使用蜂毒肽作为模型蛋白，研究了2,2,2-三氟乙醇（TFE）对水和TFE混合物中蛋白质/肽的一般作用。通过小角度X射线散射研究确定了TFE-H ₂ O混合物中的涨落和Kirkwood-Buff积分（KBI），定量描述了不均匀性，并将其与其他醇的水溶液进行了比较。混合物的浓度波动为甲醇<乙醇≪ TFE <叔-丁醇<1-丙醇，表明TFE–H ₂中分子分布的不均匀性O混合物出乎意料地与一系列单醇类产品相比。根据TFE分子之间KBI的浓度依赖性，发现TFE分子之间的强吸引力对诱导螺旋构象不一定很重要，这与先前提出的机理不一致。为了解决这个问题，通过结合KBI和实验光谱研究报告的螺旋含量，我们定量评估了TFE与蜂毒素的优先结合参数的变化，这归因于线圈-螺旋转变。结果，我们在TFE诱导的螺旋形成中发现了两种不同的机制。在TFE的约2 M以下的稀浓度区域中，TFE分子彼此之间不聚集，由于它们之间的直接相互作用，即与溶剂波动无关，TFE与螺旋之间发生了过量的优先结合。在约2 M以上的较高浓度区域中，除了前者的作用外，过量的优先结合因溶剂波动而显着增强。该方案应作为TFE对蛋白质/肽的一般助溶剂作用而持有。