Files
Abstract
Nickel superoxide dismutase (NiSOD) is a metalloenzyme that disproportionates superoxide (O2) to O2 and H2O2 by alternating between reduced and oxidized Ni states. NiSODs coordination sphere also varies depending on oxidation states. In the reduced form, Ni(II) is coordinated in square-planar geometry by the primary amine of His1, carboxamide-N of Cys2, and two thiolate-S from Cys2 and Cys6. In the oxidized form, the imidazole-N of His1 binds axially in square pyramidal geometry that has proven crucial to accessing Ni(III) and maintaining the diffusion-controlled rate of disproportionation. Given the unusual coordination environment, Ni(II/III) redox, and O2 tolerance of the Cys-S ligands, we have turned to a synthetic approach to create low molecular weight mimics of the active site and gain insight into NiSOD. Using a metallosynthon precursor, we developed NiN2S2 complexes that model reduced NiSOD and vary at one coordination site to tune the electronic environment of the system. As a result of synthetic modifications in two systems, the S character in the redox active molecular orbital was sufficiently suppressed so as to allow access to a high valent Ni state. This is the first observation of a stable Ni(III) species in NiSOD synthetic analogues with structural and electronic donors that match the enzyme. Spectroscopic and computational evidence reveal that the high valent state is a resonance species of Ni(II)-thiyl radical and Ni(III)-thiolate character. Due to this resonance, the Ni complex exhibits unique reactivity with nitric oxide (NO), resulting in reductive nitrosylation and a dimeric {NiNO}10 species bridged by thiolates and N-nitrosamines. Turning to oxidized NiSOD models, a peptide-based His-Cysteamine ligand platform was developed, which contains His-NIm poised to bind to Ni in the apical position. However, this system coordinated Ni in an unidentified manner that did not involve the carboxamido-N, as evidenced by spectroscopic characterization. Taken together, these results highlight the reactivity of the amine/carboxamide/thiolate ligand environment of NiSOD. Ni is also found in other metalloenzymes and regulatory systems, and to better understand pathways of Ni trafficking, we have developed a novel N2S2 fluorescent sensor based upon the primary coordination sphere of reduced NiSOD to detect Ni(II) in solution.