Deuterium isotope effect on the intramolecular electron transfer in Pseudomonas aeruginosa azurin

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 ΔS(‡) 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.