Self - Assembling Ionophores

interpreted by using a variety of schemes involving Ni(III)/Ni( I and Ni( I I I ) /Ni( I I ) /Ni( I ) / Ni(0)22 formal oxidation states. In all these schemes, the assignment of Ni(l1l) to the oxidized forms of the enzymes is based on the observation of hyperfine in 61Ni-labeled enzymes, the similarity of the EPR spectra to EPR spectra of tetragonal Ni(II1) coordination compounds, and the association of the signal with oxidized forms of the enzymes. The studies outlined here suggest that Ni(l1l) has no real existence in systems with simple alkyl thiolate ligation. From the viewpoint of ligand oxidation, the ”Ni(111)” EPR signal could be interpreted as arising from a tetragonal S = I , Ni(1l) center with the spin in the dXz+ orbital antiferromagnetically coupled to a thiyl radical. The resulting S = system (with an unpaired spin in the dzz orbital) is consistent with the observed EPR spectrum and with the small Ni K-edge shifts observed in XAS spectra,2ob is more consistent with known Ni thiolate redox chemistry, and suggests that nature may have selected a non-redox-actiue metal (Ni) for these roles in order to stabilize ligand-based redox chemistry.