The pKa is the standard measure used to describe the aqueous proton affinity of a compound, indicating the proton concentration (pH) at which two protonation states (e.g. A- and AH) have equal free energy. However, compounds can have additional protonation states (e.g. AH2+), and may assume multiple tautomeric forms, with the protons in different positions (microstates). Macroscopic pKas give the pH where the molecule changes its total number of protons, while microscopic pKas identify the tautomeric states involved. As tautomers have the same number of protons, the free energy difference between them and their relative probability is pH independent so there is no pKa connecting them. The question arises: What is the best way to describe protonation equilibria of a complex molecule in any pH range? Knowing the number of protons and the relative free energy of all microstates at a single pH, ∆G°, provides all the information needed to determine the free energy, and thus the probability of each microstate at each pH. Microstate probabilities as a function of pH generate titration curves that highlight the low energy, observable microstates, which can then be compared with experiment. A network description connecting microstates as nodes makes it straightforward to test thermodynamic consistency of microstate free energies. The utility of this analysis is illustrated by a description of one molecule from the SAMPL6 Blind pKa Prediction Challenge. Analysis of microstate ∆G°s also makes a more compact way to archive and compare the pH dependent behavior of compounds with multiple protonatable sites.

中文翻译：

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标准状态自由能，而不是 pKas，是描述小分子质子化和互变异构状态的理想选择。

pKa 是用于描述化合物的水性质子亲和力的标准量度，表示两种质子化状态（例如 A- 和 AH）具有相等自由能时的质子浓度 (pH)。然而，化合物可以具有额外的质子化状态（例如 AH2+），并且可以呈现多种互变异构形式，质子处于不同位置（微观状态）。宏观 pKa 给出分子改变其质子总数的 pH 值，而微观 pKa 确定所涉及的互变异构状态。由于互变异构体具有相同数量的质子，它们之间的自由能差异及其相对概率与 pH 无关，因此没有连接它们的 pKa。问题来了：在任何 pH 范围内描述复杂分子的质子化平衡的最佳方法是什么？知道质子数和所有微观状态在单一 pH 值下的相对自由能 ∆G°，提供了确定自由能所需的所有信息，从而提供了在每个 pH 值下每个微观状态的概率。微状态概率作为 pH 的函数生成滴定曲线，突出显示低能量、可观察的微状态，然后可以与实验进行比较。将微观状态连接为节点的网络描述使得测试微观状态自由能的热力学一致性变得简单。SAMPL6 盲 pKa 预测挑战中的一个分子的描述说明了该分析的实用性。