A DFT study of the mechanism of Ni superoxide dismutase (NiSOD): role of the active site cysteine-6 residue in the oxidative half-reaction

In the present DFT study, the catalytic mechanism of H2O2 formation in the oxidative half-reaction of NiSOD, E-Ni(II) + O2- + 2H+ --> E-Ni(III) + H2O2, has been investigated. The main objective of this study is to investigate the source of two protons required in this half-reaction. The proposed mechanism consists of two steps: superoxide coordination and H2O2 formation. The effect of protonation of Cys6 and the proton donating roles of side chains (S) and backbones (B) of His1, Asp3, Cys6, and Tyr9 residues in these two steps have been studied in detail. For protonated Cys6, superoxide binding generates a Ni(III)-O2H species in a process that is exothermic by 17.4 kcal/mol (in protein environment using the continuum model). From the Ni(III)-O2H species, H2O2 formation occurs through a proton donation by His1 via Tyr9, which relative to the resting position of the enzyme is exothermic by 4.9 kcal/mol. In this pathway, a proton donating role of His1 residue is proposed. However, for unprotonated Cys6, a Ni(II)-O2- species is generated in a process that is exothermic by 11.3 kcal/mol. From the Ni(II)-O2- species, the only feasible pathway for H2O2 formation is through donation of protons by the Tyr9(S)-Asp3(S) pair. The results discussed in this study elucidate the role of the active site residues in the catalytic cycle and provide intricate details of the complex functioning of this enzyme.     

Water oxidation catalyzed by a new tetracobalt-substituted polyoxometalate complex: [{Co4(μ-OH)(H2O)3}(Si2W19O70)]11-

A new polyoxometalate of earth adundant elements [{Co(4)(μ-OH)(H(2)O)(3)}(Si(2)W(19)O(70))](11-) has been synthesized, characterized and shown to be a water oxidation catalyst. The initial catalytic complex is unstable and slowly undergoes hydrolysis. The hydrolysis products have been isolated and characterized, and their catalytic water oxidation activity is assessed.     

Uniformly superparacompact,completely paracompact,and strongly paracompact uniform spaces

In this paper, we introduce and study uniform analogs of complete paracompactness, strong paracompactness, and superparacompactness. __________ Translated from Sovremennaya Matematika i Ee Prilozheniya (Contemporary Mathematics and Its Applications), Vol. 34, General Topology, 2005.     

Can the ebselen derivatives catalyze the isomerization of peroxynitrite to nitrate?

The reaction of ebselen and its derivatives (1-7) with peroxynitrite anion (ONOO(-); PN) has been studied in gas phase and in aqueous, dichloromethane, benzene, and cyclohexane solutions using B3LYP/6-311+G(d,p)//B3LYP/6-311G(d,p) and PCM-B3LYP/6-311+G(d,p)//B3LYP/6-311G(d,p) approaches, respectively. It was shown that the reaction of 2 (R=H) with PN proceeds via 2 + PN --> 2-PN --> 2-TS1 (O-O activation) --> 2-O(NO(2)(-)()) --> 2-SeO + NO(2)(-) pathway with a rate-determining barrier of 25.3 (14.8) kcal/mol at the NO(2)(-) dissociation step (numbers presented without parentheses are enthalpies, and those in parentheses are Gibbs free energies). The NO(3)(-) formation process, starting from the complex 2-O(NO(2)(-)()), requires by (7.9) kcal/mol more energy than the NO(2)(-) dissociation process and is unlikely to compete with the latter. Thus, in the gas phase, the peroxynitrite --> nitrate isomerization catalyzed by complex 2 is unlikely to occur. It is shown that the NO(3)(-) formation process is slightly more favorably than the NO(2)(-) dissociation process for complex 4, with a strongest electron-withdrawing ligand R=CF(3). Therefore, complex 4 (as well as complex 6 with R=OH) is predicted to be a good catalyst for peroxynitrite <--> nitrite isomerization in the gas phase. Solvent effects (a) change the rate-determining step of the reaction 2 + PN from NO(2)(-) dissociation in the gas phase to O-O activation, which occurs with barriers of (13.9), (8.4), (8.4), and (8.2) kcal/mol in water, dichloromethane, benzene, and cyclohexane, respectively, and (b) significantly reduce the NO(2)(-) dissociation energy, while only slightly destabilizing the NO(3)(-) formation barrier, and make the peroxynitrite <--> nitrate isomerization process practically impossible, even for complex 4.     

The dependence of the extent of C2H6 elimination on the orientation of the C2H5 group in ethylbicyclo [4.4.0]decanes

The elimination of C₂H₆ from the molecular ions of trans-fused 2- and 3-ethylbicyclo [4.4.0] decanes with the axial C₂H₅ group takes place to a greater extent than in the corresponding equatorial epimers.     

Sorption of Cr(VI) on the Anion-Exchange Fibrous Material Based on Nitron

The kinetics and thermodynamics of Cr(VI) sorption under the static and dynamic conditions on the fibrous sorbent SMA-1 prepared by chemical modification of polyacrylonitrile were studied.     

Gauge Field Fluxes and Bianchi Identities in Extended Field Theories

Theoretical and Mathematical Physics - The tensor hierarchy of exceptional field theories contains gauge fields satisfying certain Bianchi identities with sources determining the interactions with...