O. Farver et al., Deuterium isotope effect on the intramolecular electron transfer in Pseudomonas aeruginosa azurin, P NAS US, 98(8), 2001, pp. 4426-4430
Citations number
54
Categorie Soggetti
Multidisciplinary
Journal title
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
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 res
ults of voltammetric measurements: the reduction potential (E-0') of Cu2+/ at 298 K is 10 mV more positive in D2O than in H2O, The temperature depend
ence of E-0' is also different, yielding entropy changes of -57 J K-1 mol-l
in water and -84 J K-1 mol(-1) in deuterium oxide. The driving force diffe
rence of 10 mV is in keeping with the kinetic isotope effect, but the contr
ibution to DeltaS(double dagger) 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 facto
r for the electron transfer process. A slightly larger thermal protein expa
nsion in H2O than in D2O (0.001 nm K-1) is sufficient both to account for t
he activation entropy difference and to compensate for the different temper
ature dependencies of E-0'. Thus, differences in driving force and thermal
expansion appear as the most straightforward rationale for the observed iso
tope effect.