Adiabaticity of the Proton-Coupled Electron-Transfer Step in the Reduction of Superoxide Effected by Nickel-Containing Superoxide Dismutase Metallopeptide-Based Mimics

Nickel-containing superoxide dismutases (NiSODs) are bacterial metalloenzymes that catalyze the disproportionation of O₂^–. These enzymes take advantage of a redox-active nickel cofactor, which cycles between the Ni­(II) and Ni­(III) oxidation states, to catalytically disprotorptionate O₂^–. The Ni­(II) center is ligated in a square planar N₂S₂ coordination environment, which, upon oxidation to Ni­(III), becomes five-coordinate following the ligation of an axial imidazole ligand. Previous studies have suggested that metallopeptide-based mimics of NiSOD reduce O₂^– through a proton-coupled electron transfer (PCET) reaction with the electron derived from a reduced Ni­(II) center and the proton from a protonated, coordinated Ni^II–S­(H⁺)–Cys moiety. The current work focuses on the O₂^– reduction half-reaction of the catalytic cycle. In this study we calculate the vibronic coupling between the reactant and product diabatic surfaces using a semiclassical formalism to determine if the PCET reaction is proceeding through an adiabatic or nonadiabatic proton tunneling process. These results were then used to calculate H/D kinetic isotope effects for the PCET process. We find that as the axial imidazole ligand becomes more strongly associated with the Ni­(II) center during the PCET reaction, the reaction becomes more nonadiabatic. This is reflected in the calculated H/D KIEs, which moderately increase as the reaction becomes more nonadiabatic. Furthermore, the results suggest that as the axial ligand becomes less Lewis basic the observed reaction rate constants for O₂^– reduction should become faster because the reaction becomes more adiabatic. These conclusions are in-line with experimental observations. The results thus indicate that variations in the axial donor’s ability to coordinate to the nickel center of NiSOD metallopeptide-based mimics will strongly influence the fundamental nature of the O₂^– reduction process

Modulation of Luminescence by Subtle Anion–Cation and Anion−π Interactions in a Trigonal Au^I···Cu^I Complex

The trigonally coordinated [AuCu­(PPh₂py)₃]­(BF₄)₂ (<b>1</b>) crystallizes in two polymorphs and a pseudopolymorph, each of which contains a trigonally coordinated cation with short Au^I–Cu^I separations of ∼2.7 Å. Under UV illumination, these crystals luminesce different colors ranging from blue to yellow. The structures of these cations are nearly superimposable, and the primary difference resides in the relative placement of the anions and solvate molecules. As confirmed by time-dependent density functional theory calculations, it is these interactions that are responsible for the differential emission properties

Sequential Oxidations of Thiolates and the Cobalt Metallocenter in a Synthetic Metallopeptide: Implications for the Biosynthesis of Nitrile Hydratase

Cobalt nitrile hydratases (Co-NHase) contain a catalytic cobalt­(III) ion coordinated in an N₂S₃ first coordination sphere composed of two amidate nitrogens and three cysteine-derived sulfur donors: a thiolate (-SR), a sulfenate (-S­(R)­O^–), and a sulfinate (-S­(R)­O₂^–). The sequence of biosynthetic reactions that leads to the post-translational oxidations of the metal and the sulfur ligands is unknown, but the process is believed to be initiated directly by oxygen. Herein we utilize cobalt bound in an N₂S₂ first coordination sphere by a seven amino acid peptide known as SODA (ACDLPCG) to model this oxidation process. Upon exposure to oxygen, Co-SODA is oxidized in two steps. In the first fast step (seconds), magnetic susceptibility measurements demonstrated that the metallocenter remains paramagnetic, that is, Co²⁺, and sulfur K-edge X-ray absorption spectroscopy (XAS) is used to show that one of the thiolates is oxidized to sulfinate. In a second process on a longer time scale (hours), magnetic susceptibility measurements and Co K-edge XAS show that the metal is oxidized to Co³⁺. Unlike other model complexes, additional slow oxidation of the second thiolate in Co-SODA is not observed, and a catalytically active complex is never formed. The likely reason is the absence of the axial thiolate ligand. In essence, the reactivity of Co-SODA can be described as between previously described models which either quickly convert to final product or are stable in air, and it offers a first glimpse into a possible oxidation pathway for nitrile hydratase biosynthesis