Contribution of the Nuclear Field Shift to Kinetic Uranium Isotope Fractionation

Isotopic fractionation of heavy elements (e.g., >100 amu) often invokes the nuclear field shift effect, which is due to the impact of the elements' large nuclei on electron density. In particular, it has been explicitly described for uranium (U) at equilibrium and during kinetic isotope fractionation in abiotic mercury reactions. By following the fractionation of U-233, U-235, U-236 and U-238 during the enzymatic reduction of hexavalent U to tetravalent U by the bacterium Shewanella oneidensis, we provide the first direct evidence of the nuclear field shift effect during biologically controlled kinetic isotope fractionation. Here, we observed the odd-even staggering trend between fractionation factors of each isotope and their nuclear masses, and show that fractionation factors are correlated better with the nuclear volume than the mass. Additionally, by computing the relative contributions of the conventional mass-dependent effect (vibrational energy) and the mass-independent effect (nuclear field shift), we demonstrate that the experimental nuclear field shift effect is smaller than the calculated equilibrium value and that this discrepancy is responsible for the kinetic fractionation factor being lower than that predicted at equilibrium.