No abstract

The correlation-consistent composite approach ͑ccCA͒, an ab initio composite technique for computing atomic and molecular energies, recently has been shown to successfully reproduce experimental data for a number of systems. The ccCA is applied to the G3/99 test set, which includes 223 enthalpies of formation, 88 adiabatic ionization potentials, 58 adiabatic electron affinities, and 8 adiabatic proton affinities. Improvements on the original ccCA formalism include replacing the small basis set quadratic configuration interaction computation with a coupled cluster computation, employing a correction for scalar relativistic effects, utilizing the tight-d forms of the second-row correlation-consistent basis sets, and revisiting the basis set chosen for geometry optimization. With two types of complete basis set extrapolation of MP2 energies, ccCA results in an almost zero mean deviation for the G3/99 set ͑with a best value of −0.10 kcal mol −1 ͒, and a 0.96 kcal mol −1 mean absolute deviation, which is equivalent to the accuracy of the G3X model chemistry. There are no optimized or empirical parameters included in the computation of ccCA energies. Except for a few systems to be discussed, ccCA performs as well as or better than Gn methods for most systems containing first-row atoms, while for systems containing second-row atoms, ccCA is an improvement over Gn model chemistries.

A three-coordinate diketiminate-nickel(I) complex with a carbonyl ligand has been characterized using EPR and IR spectroscopies and X-ray crystallography. The T geometry (bending from the sterically favored C 2v structure) contrasts with that of isosteric d 9 copper(II) complexes. DFT calculations on a truncated model reproduce experimental geometries, implying that the geometric differences are electronic in nature. Analysis of the charge distribution in the complexes shows that the geometry of the threecoordinate d 9 complexes is affected by differential charge donation of the ligands to the metal center.Three-coordinate complexes of transition metals with partially filled d shells have received attention because of their unusual reactivity and electronic structure. 1 The predominant geometry in crystallographically characterized three-coordinate complexes is trigonal-planar, with the ligands symmetrically distributed to minimize steric effects. The main exception to this generalization is with low-spin d 8 systems, which clearly favor a T-shaped geometry. 2 In recent papers, we described the synthesis and electronic structure of a series of three-coordinate complexes with d 6 , d 7 , d 8 , and d 9 electronic configurations at the metal center. 3,4 A bulky -diketiminate ligand ("L") was used, and a sterically favored Y geometry was evident at the metal in each case. In the Y geometry, the nickel coordination environment is idealized C 2V , with the non-diketiminate ligand on both mirror planes. The d 9 example, L tBu Ni(THF) (Figure 1, left), is notable because there are few examples of isolable three-coordinate nickel(I) complexes, 5 one series of three-coordinate copper(II) complexes, 6 and no threecoordinate d 9 complexes of heavier metals. Understanding of three-coordinate nickel(I) complexes is also biologically relevant because three-coordination is potentially accessible in the low-coordinate "proximal" nickel site of acetylcoenzyme A synthase (where methylcobalamin, CO, and coenzyme A are transformed into acetyl-coenzyme A). 7 Below, we use synthetic, crystallographic, and theoretical studies to show that the first three-coordinate nickel(I) carbonyl complex prefers a T geometry. We compare it to relevant nickel(I) and copper(II) complexes to arrive at new * To whom correspondence should be addressed. E-mail: tomc@unt.edu (T.R.C.), holland@chem.rochester.edu (P.L.H. . Ellipsoids are at 50% probability, and hydrogen atoms are omitted for clarity.

Glutathione synthetase is an enzyme that belongs to the glutathione synthetase ATP-binding domain-like superfamily. It catalyzes the second step in the biosynthesis of glutathione from ␥-glutamylcysteine and glycine in an ATP-dependent manner. Glutathione synthetase has been purified and sequenced from a variety of biological sources; still, its exact mechanism is not fully understood. A variety of structural alignment methods were applied and four highly conserved residues of human glutathione synthetase (Glu-144, Asn-146, Lys-305, and Lys-364) were identified in the binding site. The function of these was studied by experimental and computational site-directed mutagenesis. The three-dimensional coordinates for several human glutathione synthetase mutant enzymes were obtained using molecular mechanics and molecular dynamics simulation techniques, starting from the reported crystal structure of human glutathione synthetase. Consistent with circular dichroism spectroscopy, our results showed no major changes to overall enzyme structure upon residue mutation. However, semiempirical calculations revealed that ligand binding is affected by these mutations. The key interactions between conserved residues and ligands were detected and found to be essential for enzymatic activity. Particularly, the negatively charged Glu-144 residue plays a major role in catalysis.

Copper nitrenes are of interest as intermediates in the catalytic aziridination of olefins and the amination of C-H bonds. However, despite advances in the isolation and study of late-transition-metal multiply bonded complexes, a bona fide structurally characterized example of a terminal copper nitrene has, to our knowledge, not been reported.In anticipation of such a report, terminal copper nitrenes are studied from a computational perspective. The nitrene complexes studied here are of the form ( -diketiminate)Cu(NPh). Density functional theory (DFT), complete active space self-consistent-field (CASSCF) electronic structure techniques, and hybrid quantum mechanical/molecular mechanical (QM/MM) methods are employed to study such species. While DFT methods indicate that a triplet (S ) 1) is the ground state, CASSCF calculations indicate that a singlet (S ) 0) is the ground state, with only a small energy gap between the singlet and triplet. Moreover, the ground-state (open-shell) singlet copper nitrene is found to be highly multiconfigurational (i.e., biradical) and to possess a bent geometry about the nitrene nitrogen, contrasting with the linear nitrene geometry of the triplet copper nitrenes. CASSCF calculations also reveal the existence of a closed-shell singlet state with some degree of multiple bonding character for the copper-nitrene bond.

With a single f-electron, Ce(III) is the simplest test case for benchmarking the thermodynamic and structural properties of hydrated Ln(III) against varying density functionals and reaction field models, in addition to determining the importance of multiconfigurational character in their wave functions. Here, the electronic structure of Ce(H2O)x(H 2O)y(3+) (x = 8, 9; y = 0, 12-14) has been examined using DFT and CASSCF calculations. The latter confirmed that the wave function of octa- and nona-aqua Ce(III) is well-described by a single configuration. Benchmarking was performed for density functionals, reaction field cavity types, and solvation reactions against the experimental free energy of hydration, DeltaG(hyd)(Ce(3+)). The UA0, UAKS, Pauling, and UFF polarized continuum model cavities displayed different performance, depending on whether one or two hydration shells were examined, and as a function of the size of the metal basis set. These results were essentially independent of the density functional employed. Using these benchmarks, the free energy for water exchange between CN = 8 and CN = 9, for which no experimental data are available, was estimated to be approximately -4 kcal/mol.

Kohlenwasserstoffe aktiviert: Isolierbare β‐Diketiminatodikupfernitren‐Komplexe wie 1, die aus [{(Cl2NN)Cu}2(μ‐Benzol)] und 1‐Adamantylazid erhalten wurden, vermitteln die Nitreninsertion in nichtaktivierte sp3‐hybridisierte C‐H‐Bindungen. In Gegenwart von 1 gelingen stöchiometrische und katalytische intermolekulare C‐H‐Aminierungen von Kohlenwasserstoffen zur Bildung von sekundären Aminen (siehe Schema). Katalysatorkonzentrationen von nur 0.05 Mol‐% können verwendet werden.

2,6-Dichlorohydroquinone 1,2-dioxygenase (PcpA) from Sphingobium chlorophenolicum ATCC 39723 is a member of a class of Fe(II)-containing hydroquinone dioxygenases that is involved in the mineralization of the pollutant pentachlorophenol. This enzyme has not been extensively characterized, despite its interesting ring-cleaving activity and use of Fe(II), which are reminiscent of the well-known extradiol catechol dioxygenases. On the basis of limited sequence homology to the extradiol catechol dioxygenases, the residues ligating the Fe(II) center were originally proposed to be H159, H227, and E276 (Xu et al. in Biochemistry 38:7659-7669, 1999). However, PcpA has higher sequence homology to a newly reported, crystallographically characterized zinc metalloenzyme that has a similar predicted fold. We generated a homology model of the structure of PcpA based upon the structure of this zinc metalloenzyme. The homology model predicts that the tertiary structure of PcpA differs significantly from that of the extradiol dioxygenases, and that the residues ligating the Fe(II) are H11, H227, and E276. This structural model was tested by mutating each of H11, H159, H227, and E276 to alanine. An additional residue that is predicted to lie near the active site and is conserved among PcpA, its closest homologues, and the extradiol dioxygenases, Y266, was mutated to phenylalanine. Of these mutants, only H159A retained significant activity, thus confirming the active-site location predicted by the homology-based structural model. The model provides an important basis for understanding the origin of the unique function of PcpA.