The density functional calculations were used to explore the dissociation energies of [N(CH3)4]+ and [P(CH3)4]+ and their derivatives from substitution of H for CH3.The results show that the dissociation energies of C—N and C—P bonds gradually increase as the number of hydrogen atoms bonded to N or P increases in the derivatives,showing a remarkable effect of the intramolecular structural environment on the dissociation of the bonds.This dependence of bond dissociation energies on the local structural environment can be ascribed to the hyperconjugation interactions between the C—H bond and lone single electron of N or P.On the basis of NBO analyses,the bonding properties of dissociated fragments and their effects on dissociation energies were discussed.
The catalytic oxidation of CO to CO2 by carbon monoxide dehydrogenases has been explored theoretically, and a large C-cluster model including the metal core [Ni-4Fe-4S] and surrounding residues and crystal water molecules was used in density functional calculations. The key species involved in the oxidation of CO at the C-cluster, Cred1, Cred2 and Cint, have been elucidated. On the basis of computational results, the plausible enzymatic mechanism for the CO oxidation was proposed. In the catalytic reaction, the first proton abstraction from the Fe（1）-bound water leads to a precursor to accommodate CO binding and the subsequently consecutive proton transfers from the metal-bound carboxylate to the amino acid residues facilitate the release of CO2. The hydrogen-bond network around the C-cluster formed by conserved residues His93, His96, Glu299, Lys563, and four water molecules in the active domain plays an important role in proton transfer and intermediate stabilization. Predicted geometries of key species show good agreement with the reported crystal structures.
