The retro-aldol mechanism in the pyrolysis kinetics of primary,secondary, and tertiary β-hydroxy ketones in the gas phase

The pyrolysis kinetics of primary, secondary, and tertiary β-hydroxy ketones have been studied in static seasoned vessels over the pressure range of 21–152 torr and the temperature range of 190°–260°C. These eliminations are homogeneous, unimolecular, and follow a first-order rate law. The rate coefficients are expressed by the following equations: for 1-hydroxy-3-butanone, log k₁(s^?1) = (12.18 ± 0.39) ? (150.0 ± 3.9) kJ mol^?1 (2.303RT)^?1; for 4-hydroxy-2-pentanone, log k₁(s^?1) = (11.64 ± 0.28) ? (142.1 ± 2.7) kJ mol^?1 (2.303RT)^?1; and for 4-hydroxy-4-methyl-2-pentanone, log k₁(s^?1) = (11.36 ± 0.52) ? (133.4 ± 4.9) kJ mol^?1 (2.303RT)^?1. The acid nature of the hydroxyl hydrogen is not determinant in rate enhancement, but important in assistance during elimination. However, methyl substitution at the hydroxyl carbon causes a small but significant increase in rates and, thus, appears to be the limiting factor in a retroaldol type of mechanism in these decompositions. © John Wiley & Sons, Inc.     

Mechanism for room-temperature single-atom lateral manipulations on semiconductors using dynamic force microscopy

Vacancy-mediated lateral manipulations of intrinsic adatoms of the Si(111)-(7x7) surface at room temperature are reported. The topographic signal during the manipulation combined with force spectroscopy measurements reveals that these manipulations can be ascribed to the so-called pulling mode, and that the Si adatoms were manipulated in the attractive tip-surface interaction regime at the relatively low short-range force value associated to the manipulation set point. First-principles calculations reveal that the presence of the tip induces structural relaxations that weaken the adatom surface bonds and manifests in a considerable local reduction of the natural diffusion barriers to adjacent adsorption positions. Close to the short-range forces measured in the experiments, these barriers are lowered near the limit that enables a thermally activated hopping at room temperature.     

Relative Permeabilities for Strictly Hyperbolic Models of Three-Phase Flow in Porous Media

Traditional mathematical models of multiphase flow in porous media use a straightforward extension of Darcys equation. The key element of these models is the appropriate formulation of the relative permeability functions. It is well known that for one-dimensional flow of three immiscible incompressible fluids, when capillarity is neglected, most relative permeability models used today give rise to regions in the saturation space with elliptic behavior (the so-called elliptic regions). We believe that this behavior is not physical, but rather the result of an incomplete mathematical model. In this paper we identify necessary conditions that must be satisfied by the relative permeability functions, so that the system of equations describing three-phase flow is strictly hyperbolic everywhere in the saturation triangle. These conditions seem to be in good agreement with pore-scale physics and experimental data.     

Nickel‐Catalyzed Enantioselective C?C Bond Formation through C?O Cleavage in Aryl Esters

We report the first enantioselective C C bond formation through C O bond cleavage using aryl ester counterparts. This method is characterized by its wide substrate scope and results in the formation of quaternary stereogenic centers with high yields and asymmetric induction.     

Symmetry calculation for molecules and transition states

The symmetry of molecules and transition states of elementary reactions is an essential property with important implications for computational chemistry. The automated identification of symmetry by computers is a very useful tool for many applications, but often relies on the availability of three‐dimensional coordinates of the atoms in the molecule and hence becomes less useful when these coordinates are a priori unavailable. This article presents a new algorithm that identifies symmetry of molecules and transition states based on an augmented graph representation of the corresponding structures, in which both topology and the presence of stereocenters are accounted for. The automorphism group order of the graph associated with the molecule or transition state is used as a starting point. A novel concept of label‐stereoisomers, that is, stereoisomers that arise after labeling homomorph substituents in the original molecule so that they become distinguishable, is introduced and used to obtain the symmetry number. The algorithm is characterized by its generic nature and avoids the use of heuristic rules that would limit the applicability. The calculated symmetry numbers are in agreement with expected values for a large and diverse set of structures, ranging from asymmetric, small molecules such as fluorochlorobromomethane to highly symmetric structures found in drug discovery assays. The new algorithm opens up new possibilities for the fast screening of the degree of symmetry of large sets of molecules. © 2014 Wiley Periodicals, Inc.     

Analysis of sulfated peptides from the skin secretion of the Pachymedusa dacnicolor frog using IMAC-Ga enrichment and high-resolution mass spectrometry

Immobilized metal ion affinity chromatography (IMAC) has been widely used for the enrichment of phosphopeptides, whereas no report exists describing the use of IMAC columns for the enrichment of sulfopeptides. In this study, we used IMAC-Ga microcolumns for the enrichment of sulfopeptides from a complex mixture of peptides, extracted from skin secretions of the Pachymedusa dacnicolor frog. The enriched fraction obtained by IMAC-Ga was analyzed by liquid chromatograpy/electrospray ionization mass spectrometry (LC/ESI-MS) in an Orbitrap XL and by matrix-assisted laser desorption/ionization time-of-flight time-of-flight (MALDI-TOF/TOF) in an ABI 4800 instrument. From this fraction, different sulfated and non-sulfated peptides belonging to the caerulin and bradykinin families were structurally characterized. Other interesting negatively charged groups, such as phosphate adducts of dermaseptins and pyridoxal phosphate attached to a protease inhibitor, were also characterized. Unexpectedly, some dermaseptin antimicrobial peptides were also enriched by IMAC-Ga and a Sauvatine-like peptide was also fully sequenced. Furthermore, neutral loss of sulfated peptides and their fragmentation patterns in the gas phase were also compared using collision-induced dissociation (CID) and high-energy collision dissociation (HCD). Our present study provides evidence that IMAC-Ga enrichment is a fast, useful and promising method for high-throughput analysis of sulfated-peptides, since high-resolution mass spectrometers can be used for this purpose.     

Effects of peptide density and elution pH on affinity chromatographic purification of human immunoglobulins A and M

A family of linear hexamer peptide ligands HWRGWV, HYFKFD and HFRRHL, initially identified for their affinity to the Fc portion of human immunoglobulin G (hIgG), also have potential for use in the purification of human immunoglobulins A (hIgA) and M (hIgM). HWRGWV demonstrated the strongest binding affinity to hIgM, followed by hIgA and hIgG respectively. The effects of N-terminal acetylation of the peptide, as well as elution buffer pH, on the chromatographic elution of human IgG, IgA and IgM from HWRGWV resins at various peptide densities (0.04-0.55 meq/g) were investigated. Over 80% recovery and 90% purity were achieved for human IgG and IgA isolation from complete minimum essential medium (cMEM) using HWRGWV resin at optimum peptide densities. For human IgM, 75.7% recovery and 86.0% purity were achieved by using HWRGWV at a low peptide density of 0.04 meq/g. Although HYFKFD and HFRRHL exhibited their ability for isolation of human IgG, IgA and IgM from cMEM as well, HWRGWV is the best option among them for large-scale purification of human IgG, IgA and IgM based on conditions tested.     

Tuning the spin-transition properties of pyrene-decorated 2,6-bispyrazolylpyridine based Fe(II) complexes

Two 2,6-bispyrazolylpyridine ligands (bpp) were functionalized with pyrene moieties through linkers of different lengths. In the ligand 2,6-di(1H-pyrazol-1-yl)-4-(pyren-1-yl)pyridine (L1) the pyrene group is directly connected to the bpp moiety via a C-C single bond, while in the ligand 4-(2,6-di(1H-pyrazol-1-yl)pyridin-4-yl)benzyl-4-(pyren-1-yl)butanoate (L2) it is separated by a benzyl ester group involving a flexible butanoic chain. Subsequent complexation of Fe(II) salts revealed dramatic the influence of the nature of the pyrene substitution on the spin-transition behaviour of the resulting complexes. Thus, compound [Fe(L1)(2)](ClO(4))(2) (1) is blocked in its high spin state due to constraints caused by a strong intermolecular π-π stacking in its structure. On the other hand, the flexible chain of ligand L2 in compounds [Fe(L2)(2)](ClO(4))(2) (2) and [Fe(L2)(2)](BF(4))(2)·CH(3)CN·H(2)O (3) prevents structural constraints allowing for reversible spin transitions. Temperature-dependent studies of the photophysical properties of compound 3 do not reveal any obvious correlation between the fluorescence of the pyrene group and the spin state of the spin transition core.     

A natural product-like inhibitor of NEDD8-activating enzyme

The natural product-like 6,6″-biapigenin has been identified as only the second inhibitor of NEDD8-activating enzyme using virtual screening. This compound was active in enzyme and cell-based assays, with potency in the micromolar range.