在非极性溶剂中，极性和非极性部分的作用可以颠倒吗？
Can the roles of polar and non-polar moieties be reversed in non-polar solvents?
摘要：
利用热力学积分方法，研究了18种氨基酸侧链等价物在不同极性溶剂（从极性最大的水到非极性最大的环己烷）中的溶剂化自由能。氨基酸侧链等价物是通过用氢原子取代骨架部分，并去除具有特殊性质的脯氨酸和甘氨酸，从20种天然氨基酸中获得的。的相对溶剂化自由能的详细分析表明，它是如何可能实现一个强大的和明确的疏水规模的氨基酸。通过区分熵和焓项的相对贡献，我们发现在水和乙醇中具有很强的负相关性，与众所周知的熵-焓补偿相关，而在环己烷中的相关性大大降低。这表明，一般来说，极性和非极性部分的作用在非极性溶剂中不能逆转。我们的研究结果与过去的实验以及数值结果进行了比较，并可能进一步阐明水作为生物溶剂的独特作用。
英文摘要：
Using thermodynamics integration, we study the solvation free energy of 18 amino acid side chain equivalents in solvents with different polarity, ranging from the most polar water to the most non-polar cyclohexane. The amino acid side chain equivalents are obtained from the 20 natural amino acids by replacing the backbone part with a hydrogen atom, and discarding proline and glycine that have special properties. A detailed analysis of the relative solvation free energies suggests how it is possible to achieve a robust and unambiguous hydrophobic scale for the amino acids. By discriminating the relative contributions of the entropic and enthalpic terms, we find strong negative correlations in water and ethanol, associated with the well-known entropy-enthalpy compensation, and a much reduced correlation in cyclohexane. This shows that in general the role of the polar and non-polar moieties cannot be reversed in a non-polar solvent. Our findings are compared with past experimental as well as numerical results, and may shed additional light on the unique role of water as biological solvent.
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