Deuterium Isotope Effect on the Intramolecular Electron Transfer in Pseudomonas aeruginosa Azurin

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 98; no. 8; pp. 4426 - 4430
• Main Authors: Farver, Ole, Zhang, Jingdong, Chi, Qijin, Pecht, Israel, Ulstrup, Jens
• Format: Journal Article
• Language: English
• Published: United States National Academy of Sciences 10.04.2001; National Acad Sciences
• Subjects: azurin; Azurin - chemistry; Azurin - metabolism; Bacteria; Biological Sciences; Cantors; Chemistry; Deuterium; Electrochemistry; Electron transfer; Electron Transport; Electrons; Entropy; isotope effect; Isotope effects; Isotopes; Kinetics; Molecules; Protons; Pseudomonas aeruginosa; Pseudomonas aeruginosa - chemistry; Solvation; Solvents; Thermodynamics
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.071043798

Intramolecular electron transfer in azurin in water and deuterium oxide has been studied over a broad temperature range. The kinetic deuterium isotope effect, kH/kD, is smaller than unity (0.7 at 298 K), primarily caused by the different activation entropies in water (-56.5 J K-1mol-1) and in deuterium oxide (-35.7 J K-1mol-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 (E0′ ) of Cu2+/+at 298 K is 10 mV more positive in D2Othan in H2O. The temperature dependence of E0′ is also different, yielding entropy changes of -57 J K-1mol-1in water and -84 J K-1mol-1in deuterium oxide. The driving force difference of 10 mV is in keeping with the kinetic isotope effect, but the contribution to Δ S‡from the temperature dependence of E0′ 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 H2Othan in D2O(0.001 nm K-1) is sufficient both to account for the activation entropy difference and to compensate for the different temperature dependencies of E0′ . Thus, differences in driving force and thermal expansion appear as the most straightforward rationale for the observed isotope effect.