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The novel pH dependences of both the activation process and catalysis by activated Achromobacter cycloclastes [N.sub.2]OR, which has indicated that the enzyme might use multiple mechanisms for [N.sub.2]O reduction, are presented. The results have shown that the transition state is stabilized by H-bonding interactions between the active site and an [N.sub.2]O-derived intermediate bound to the catalytic [Cu.sub.z] cluster.

The nickel (I) complex [PhTt(super Ad]Ni(CO) (PhTt(super Ad)), phenyltris((1-adamantylthio)methyl)borate) reacts with O(sub 2) generating a 1:1 species identified as a side-on dioxygen adduct based on its spectroscopic properties as supported by DFT computational results and by its reactivity. The successful pursuit of the strategy, which led to the discovery of a side-on dioxygen complex of nickel, is reported.

Research has been conducted on Ni (super)+ four-coordinate complex. Results demonstrate that this complex serves as a Ni-CO reaction intermediate in the acetyl-CoA synthase catalytic cycle.

Abstract: Pseudoazurin (PAz), a well-characterized blue copper electron-transfer protein, is shown herein to be capable of mediating electron transfer to the nitrous oxide reductase (N2OR) from Achromobacter cycloclastes (Ac). Spectroscopic measurements demonstrate that reduced PAz is efficiently re-oxidized by a catalytic amount of N2OR in the presence of N2O. Fits of the kinetics resulted in K M (N2O) and k cat values of 19.1±3.8μM and 89.3±4.2s−1 respectively. The K M (PAz) was 28.8±6.6μM. The electrochemistry of Ac pseudoazurin (AcPAz) in the presence of Ac nitrous oxide reductase (AcN2OR) and N2O displayed an enhanced cathodic sigmoidal current–potential curve, in excellent agreement with the re-oxidation of reduced AcPAz during the catalytic reduction of N2O by AcN2OR. Modeling the structure of the AcPAz–AcN2OR electron transfer complex indicates that AcPAz binds near CuA in AcN2OR, with parameters consistent with the formation of a transient, weakly-bound complex. Multiple, potentially efficient electron-transfer pathways between the blue-copper center in AcPAz and CuA were also identified. Collectively, the data establish that PAz is capable of donating electrons to N2OR in N2O reduction and is a strong candidate for the physiological electron donor to N2OR in Ac. [Copyright &y& Elsevier]

Nitrous oxide reductase (N2OR) from Achromobacter cycloclastes (Ac) can be reductively activated with reduced methyl viologen over a broad range of pH. Activated Ac N2OR displays a complex activity profile as a function of the pH at which catalytic turnover is measured. Spectroscopic and steady-state kinetics data suggest that [H+] has multiple effects on both the activation and the catalytic reactions. These experimental results are in good agreement with previous theoretical studies, which suggested that the transition state is stabilized by H-bonding interactions between the active site and an N2O-derived intermediate bound to the catalytic Cuz cluster. [ABSTRACT FROM AUTHOR]

The nickel(I) complex [PhTtAd]Ni(CO) (PhTtAd, phenyltris((1-adamantylthio)methyl)borate reacts with O2 generating a 1:1 species identified as a side-on dioxygen adduct based on its spectroscopic properties as supported by DFT computational results and by its reactivity. The Ni EXAFS data are fit to a S2O2 coordination environment with short Ni-O distances, 1.85 Å. The brown complex displays a rhombic EPR signal with g values of .24, 2.19, 2.01. DFT and INDO/S-CI computations replicate the EXAFS and EPR features and suggest that 2 is a side-on [NiO2]+ complex with geometric and electronic properties that are best rationalized in terms of a highly covalent Ni(II)-suprexo description. (PhTtAd]-Ni(O2) oxidizes PPh3 to OPPh3-, and [PhTttBu]Ni(CO) to the nonsymmetric [PhTtAdNi(μ-O) 2Ni[PhTttBu]dimer. [ABSTRACT FROM AUTHOR]

The four-coordinate Ni(+) complex [PhTt(t)(Bu)]Ni(I)CO, where PhTt(t)()(Bu) = phenyltris((tert-buthylthio)methyl)borate (a tridentate thioether donor ligand), serves as a possible model for key Ni-CO reaction intermediates in the acetyl-CoA synthase (ACS) catalytic cycle. Resonance Raman, electronic absorption, magnetic circular dichroism (MCD), variable-temperature variable-field MCD, and electron paramagnetic resonance spectroscopies were utilized in conjunction with density functional theory and semiemperical INDO/S-CI calculations to investigate the ground and excited states of [PhTt(t)()(Bu)]Ni(I)CO. These studies reveal extensive Ni(+) --> CO pi-back-bonding interactions, as evidenced by a low C-O stretching frequency (1995 cm(-)(1)), a calculated C-O stretching force constant of 15.5 mdyn/A (as compared to k(CO)(free CO) = 18.7 mdyn/A), and strong Ni(+) --> CO charge-transfer absorption intensities. Calculations reveal that this high degree of pi-back-bonding is due to the fact that the Ni(+) 3d orbitals are in close energetic proximity to the CO pi acceptor orbitals. In the ACS "paramagnetic catalytic cycle", the high degree of pi-back-bonding in the putative Ni(+)-CO intermediate (the NiFeC species) is not expected to preclude methyl transfer from CH(3)-CoFeSP.