A not so simple local multiplier algebra

We construct an AF-algebra A such that its local multiplier algebra Mloc(A) does not agree with Mloc(Mloc(A)), thus answering a question raised by G.K. Pedersen in 1978.     

On the Existence of a Pyramidality Effect in d(8).d(8) Contacts. Theoretical Study and Structural Correlation

A qualitative molecular orbital study of d(8).d(8) contacts, using as models the dimers of [RhCl(CO)(3)] and cis-[PtCl(2)(CO)(2)], is presented, focusing on the effect of the pyramidalization of the metal atom on the strength of the metal-metal interaction. The existence of a pyramidality effect in dimers of d(8)-ML(4) complexes is supported also by the results of ab initio MP2 calculations on the dimer of cis-[PtCl(2)(CO)(2)]. A structural database analysis shows that a correlation between the pyramidality and the M.M distance exists for several families of dimers, oligomers, and polymers of square planar complexes of Ni(II), Pd(II), Pt(II), and Au(III). The importance of the pyramidality effect in those compounds is discussed in comparison with similar effects in binuclear complexes with metal-metal bonds of different multiplicities.     

Ligand effects and dimer formation in dicoordinated copper(I) complexes

The dimers of dicoordinated Cu(I) complexes of the type [CuXL], where X^? = Cl^?, Br^?, I^?, or CN^? and L is a neutral ligand such as NH₃, PH₃, or CNCH₃, have been studied by means of ab initio and density functional calculations. The performance of density functional calculations using the hybrid B3LYP method has been compared with MP2 results. Ligand and conformational effects are analyzed for two types of dimers, notably those in which the monomers retain their linear conformation and associate through Cu–Cu contacts and those in which the X ligands act as bridges resulting in tricoordination of each Cu atom and a rhombic Cu₂X₂ core. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001     

Theoretical study of the structure and vibrational spectra of the (H2O)2…HF and H2O…(HF)2 molecular complexes

Using the Hartree–Fock and MP 2 methods with bases of up to 6-31++G (2d, 2p) quality, the optimum geometry of the 1:2 and 2:1 (H₂O)_n… (HF)_m complexes of water and hydrogen fluoride is searched in a systematic way. Two minimum-energy conformations are found for the 1:2 complex connected through a low-energy transition state. For the 2:1 complex, only one minimum-energy structure is obtained. The analysis of the geometries of the minima and their vibrational frequencies shows that none of them can be used to explain the existence of the H …F? H reverse complex detected experimentally. © 1994 John Wiley & Sons, Inc.     

Long‐Range Effects in Anion–π Interactions: Their Crucial Role in the Inhibition Mechanism of Mycobacterium Tuberculosis Malate Synthase

The glyoxylate shunt is an anaplerotic bypass of the traditional Krebs cycle. It plays a prominent role in Mycobacterium tuberculosis virulence, so it can be exploited for the development of antitubercular therapeutics. The shunt involves two enzymes: isocitrate lyase (ICL) and malate synthase (GlcB). The shunt bypasses two steps of the tricarboxylic acid cycle, allowing the incorporation of carbon, and thus, refilling oxaloacetate under carbon‐limiting conditions. The targeting of ICL is complicated; however, GlcB, which accommodates the pantothenate tail of acetyl‐CoA in the active site, is easier to target. A catalytic Mg²⁺ unit is located at the bottom of the cavity, and plays a very important role. Recently, the development of effective antituberculosis drugs based on phenyldiketo acids (PDKAs) has been reported. Interestingly, all the crystal structures of GlcB–inhibitor complexes exhibit close contact between the carboxylate of Asp633 and the face of the aromatic ring of the inhibitor. Remarkably, the replacement of the phenyl ring in PDKA by aliphatic moieties yields inactive inhibitors, suggesting that the aromatic moiety is crucial for inhibition. However, the aromatic ring of PDKA is not electron‐deficient, and consequently, the anion–π interaction is expected to be very weak (dominated only by polarization effects). Herein, through a combination analysis of the recent X‐ray structures of GlcB–PDKA complexes retrieved from the protein data bank (PDB) and computational ab initio studies (RI‐MP2/def2‐TZVP level of theory), we demonstrate the prominent role of the Mg²⁺ ion in the active site, which promotes long‐range enhancement of the anion–π interaction.     

Engineering the Donor Selectivity of D‐Fructose‐6‐Phosphate Aldolase for Biocatalytic Asymmetric Cross‐Aldol Additions of Glycolaldehyde

D ‐Fructose‐6‐phosphate aldolase (FSA) is a unique catalyst for asymmetric cross‐aldol additions of glycolaldehyde. A combination of a structure‐guided approach of saturation mutagenesis, site‐directed mutagenesis, and computational modeling was applied to construct a set of FSA variants that improved the catalytic efficiency towards glycolaldehyde dimerization up to 1800‐fold. A combination of mutations in positions L107, A129, and A165 provided a toolbox of FSA variants that expand the synthetic possibilities towards the preparation of aldose‐like carbohydrate compounds. The new FSA variants were applied as highly efficient catalysts for cross‐aldol additions of glycolaldehyde to N‐carbobenzyloxyaminoaldehydes to furnish between 80–98 % aldol adduct under optimized reaction conditions. Donor competition experiments showed high selectivity for glycolaldehyde relative to dihydroxyacetone or hydroxyacetone. These results demonstrate the exceptional malleability of the active site in FSA, which can be remodeled to accept a wide spectrum of donor and acceptor substrates with high efficiency and selectivity.