Baldwin R L, Muller N
Department of Biochemistry, Stanford University School of Medicine, CA 94305.
Proc Natl Acad Sci U S A. 1992 Aug 1;89(15):7110-3. doi: 10.1073/pnas.89.15.7110.
A challenge in understanding the thermodynamics of protein unfolding is to explain the 1979 puzzle posed by Privalov. Why do values of the specific enthalpy and specific entropy of unfolding both converge to common values at approximately the same temperature (Th* approximately equal to Ts*) when extrapolated linearly versus temperature? In 1986, a liquid hydrocarbon model gave an explanation for convergence of the specific entropies at Ts*: it happens because the contribution of the hydrophobic effect to the entropy of unfolding goes to zero at Ts*. The reason for convergence of the specific enthalpies at Th* and for the equality Th* approximately equal to Ts* has remained, however, a matter for speculation; recently, some explanations have been given that are based on models for polar interactions in protein folding. We show here that the relation Th* approximately equal to Ts* can be derived straightforwardly without making any assumptions either about polar interactions or about splitting the hydrophobic interaction into two terms--one for the "hydrophobic hydration" and the other for the residual effect, as suggested recently. Thus, the liquid hydrocarbon model explains both halves of Privalov's puzzle. A similar conclusion has been reached independently by A. Doig and D. H. Williams (personal communication). It has been proposed recently that a correction should be made for the relative sizes of a hydrocarbon solute and water when computing the thermodynamic properties of the hydrophobic interaction from a solvent transfer experiment. This correction affects the temperature at which the entropy of transfer equals zero, and it is important to evaluate its effect on the convergence temperature Ts*. We show that making the size correction does not change the conclusion, reached earlier, that the liquid hydrocarbon model explains the convergence of the specific entropies of protein unfolding.
理解蛋白质解折叠热力学的一个挑战是解释普里瓦洛夫在1979年提出的谜题。当相对于温度进行线性外推时，为什么解折叠的比焓值和比熵值在大约相同的温度（Th* 近似等于 Ts*）下都收敛到共同的值？1986年，一个液态烃模型对Ts* 时比熵的收敛给出了解释：之所以会这样，是因为疏水效应解折叠熵的贡献在Ts* 时变为零。然而，Th* 时比焓收敛以及Th* 近似等于 Ts* 的原因仍然是一个推测的问题；最近，已经给出了一些基于蛋白质折叠中极性相互作用模型的解释。我们在此表明，Th* 近似等于 Ts* 这一关系可以直接推导出来，而无需对极性相互作用或像最近所建议的那样将疏水相互作用拆分为两项（一项用于“疏水水合”，另一项用于残余效应）做出任何假设。因此，液态烃模型解释了普里瓦洛夫谜题的两个方面。A. 多伊格和D. H. 威廉姆斯（个人交流）独立得出了类似的结论。最近有人提出，在从溶剂转移实验计算疏水相互作用的热力学性质时，应该对烃溶质和水的相对大小进行校正。这种校正会影响转移熵等于零的温度，评估其对收敛温度Ts* 的影响很重要。我们表明，进行大小校正并不会改变早先得出的结论，即液态烃模型解释了蛋白质解折叠比熵的收敛。
