Dixon H B, Clarke S D, Smith G A, Carne T K
Department of Biochemistry, University of Cambridge, U.K.
Biochem J. 1991 Aug 15;278 ( Pt 1)(Pt 1):279-84. doi: 10.1042/bj2780279.
An acid, HnA, with n ionizing groups is known to have the same titration curve as an equimolar mixture of n hypothetical monobasic acids, whose dissociation constants are known as the 'titration constants' of the real acid. We show that the pH-dependence of any property of HnA is also represented by the sum of one-site titration curves, characterized by these same titration constants. Since one such property is the degree of dissociation of one of the dissociating groups, a fraction of each group shows each of the various titration pK values, so that the group partitions among them. The n groups therefore share the same n titration pK values but differ in the fractions belonging to each. The one H+ ion per molecule that titrates with each pK is thus made up of the fractions, one from each group, that share this pK value. A group may possess a single pK value, in that it contributes virtually all of this pK and almost nothing to the others, only if either (1) in titrates in a different pH range from the other groups or (2) its affinity for H+ is unaffected by their ionization state.
已知具有n个可电离基团的酸HnA，其滴定曲线与n种假设的一元酸的等摩尔混合物相同，这些一元酸的解离常数被称为该实际酸的“滴定常数”。我们表明，HnA任何性质的pH依赖性也由以这些相同滴定常数为特征的单位点滴定曲线之和表示。由于这样一种性质是其中一个解离基团的解离度，每个基团的一部分表现出各种滴定pK值中的每一个，因此基团在它们之间进行分配。因此，n个基团共享相同的n个滴定pK值，但属于每个pK值的分数不同。因此，与每个pK值滴定的每个分子中的一个H⁺离子由每个基团中共享该pK值的分数组成。一个基团可能具有单个pK值，即它几乎贡献了所有这个pK值，而对其他pK值几乎没有贡献，前提是要么（1）它在与其他基团不同的pH范围内滴定，要么（2）它对H⁺的亲和力不受它们电离状态的影响。
