反应pH依赖性曲线的形状。
Shapes of curves of pH-dependence of reactions.
作者信息
Dixon H B
出版信息
Biochem J. 1973 Jan;131(1):149-54. doi: 10.1042/bj1310149.
A simple case is considered in which the rate of a two-step reaction depends on pH because the intermediate formed in the first step has to gain (or lose) a proton before it can react in the second step, and in which the rate-determining step therefore changes with pH. The curves of reaction rate against pH are shown to be symmetrical, and the sharpest peak possible has a width at half its height of 1.53pH units, i.e. of 2log(3+2 radical2). Any particular curve for this situation proves to be identical with a curve that could be generated for the pH-dependence of a single-step reaction in which the rate is proportional to the concentration of a particular ionic form of a reactant. Curves for the latter situation, however, can have forms impossible for the former case in which the rate-determining step changes, but only if the protonations that activate and deactivate the reactant are co-operative. The peak can then become even sharper, and its width at half its height can fall to 1.14pH units, i.e. to 2log(2+ radical3).
考虑一个简单的例子，其中一个两步反应的速率取决于pH值，因为第一步形成的中间体在第二步反应之前必须获得（或失去）一个质子，因此决定反应速率的步骤会随pH值变化。反应速率与pH值的曲线显示为对称的，可能出现的最尖锐峰在其高度一半处的宽度为1.53个pH单位，即2log(3 + 2√2)。这种情况下的任何特定曲线都被证明与单步反应的pH依赖性曲线相同，在单步反应中，反应速率与反应物特定离子形式的浓度成正比。然而，后一种情况的曲线可能具有前一种决定反应速率步骤发生变化的情况所不可能有的形式，但前提是激活和失活反应物的质子化是协同的。此时峰可能会变得更尖锐，其在高度一半处的宽度可能降至1.14个pH单位，即2log(2 + √3)。
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