Farver O, Zhang J, Chi Q, Pecht I, Ulstrup J
Institute of Analytical and Pharmaceutical Chemistry, The Royal Danish School of Pharmacy, DK-2100 Copenhagen Ø, Denmark.
Proc Natl Acad Sci U S A. 2001 Apr 10;98(8):4426-30. doi: 10.1073/pnas.071043798. Epub 2001 Apr 3.
Intramolecular electron transfer in azurin in water and deuterium oxide has been studied over a broad temperature range. The kinetic deuterium isotope effect, k(H)/k(D), is smaller than unity (0.7 at 298 K), primarily caused by the different activation entropies in water (-56.5 J K(-1) mol(-1)) and in deuterium oxide (-35.7 J K(-1) mol(-1)). This difference suggests a role for distinct protein solvation in the two media, which is supported by the results of voltammetric measurements: the reduction potential (E(0')) of Cu(2+/+) at 298 K is 10 mV more positive in D(2)O than in H(2)O. The temperature dependence of E(0') is also different, yielding entropy changes of -57 J K(-1) mol(-1) in water and -84 J K(-1) mol(-1) in deuterium oxide. The driving force difference of 10 mV is in keeping with the kinetic isotope effect, but the contribution to DeltaS from the temperature dependence of E(0') is positive rather than negative. Isotope effects are, however, also inherent in the nuclear reorganization Gibbs free energy and in the tunneling factor for the electron transfer process. A slightly larger thermal protein expansion in H(2)O than in D(2)O (0.001 nm K(-1)) is sufficient both to account for the activation entropy difference and to compensate for the different temperature dependencies of E(0'). Thus, differences in driving force and thermal expansion appear as the most straightforward rationale for the observed isotope effect.
在很宽的温度范围内研究了水和重水中天青蛋白的分子内电子转移。动力学氘同位素效应k(H)/k(D)小于1（298 K时为0.7），主要是由水（-56.5 J K⁻¹ mol⁻¹）和重水（-35.7 J K⁻¹ mol⁻¹）中不同的活化熵引起的。这种差异表明在两种介质中存在不同的蛋白质溶剂化作用，这得到了伏安测量结果的支持：298 K时Cu(2+/+)在D₂O中的还原电位（E(0')）比在H₂O中更正10 mV。E(0')的温度依赖性也不同，在水中产生的熵变为-57 J K⁻¹ mol⁻¹，在重水中为-84 J K⁻¹ mol⁻¹。10 mV的驱动力差异与动力学同位素效应一致，但E(0')的温度依赖性对ΔS的贡献是正的而非负的。然而，同位素效应也存在于核重组吉布斯自由能和电子转移过程的隧穿因子中。H₂O中蛋白质的热膨胀略大于D₂O中（0.001 nm K⁻¹），这足以解释活化熵的差异并补偿E(0')不同的温度依赖性。因此，驱动力和热膨胀的差异似乎是观察到的同位素效应最直接的原因。
