General rules for predicting where a catalytic reaction should occur on metal surfaces: A density functional theory study of C-H and C-O bond breaking/making on flat, stepped, and kinked metal surfaces

To predict where a catalytic reaction should occur is a fundamental issue scientifically. Technologically, it is also important because it can facilitate the catalyst's design. However, to date, the understanding of this issue is rather limited. In this work, two types of reactions, CH4 CH3 + H and CO C + 0 on two transition metal surfaces, were chosen as model systems aiming to address in general where a catalytic reaction should occur. The dissociations of CH4 - CH3 + H and CO --> C + O and their reverse reactions on flat, stepped, and kinked Rh and Pd surfaces were studied in detail. We find the following: First, for the CH4 Ch(3) + H reaction, the dissociation barrier is reduced by similar to0.3 eV on steps and kinks as compared to that on flat surfaces. On the other hand, there is essentially no difference in barrier for the association reaction of CH3 + H on the flat surfaces and the defects. Second, for the CO C + 0 reaction, the dissociation barrier decreases dramatically (more than 0.8 eV on Rh and Pd) on steps and kinks as compared to that on flat surfaces. In contrast to the CH3 + H reaction, the C + 0 association reaction also preferentially occurs on steps and kinks. We also present a detailed analysis of the reaction barriers in which each barrier is decomposed quantitatively into a local electronic effect and a geometrical effect. Our DFT calculations show that surface defects such as steps and kinks can largely facilitate bond breaking, while whether the surface defects could promote bond formation depends on the individual reaction as well as the particular metal. The physical origin of these trends is identified and discussed. On the basis of our results, we arrive at some simple rules with respect to where a reaction should occur: (i) defects such as steps are always favored for dissociation reactions as compared to flat surfaces; and (ii) the reaction site of the association reactions is largely related to the magnitude of the bonding competition effect, which is determined by the reactant and metal valency. Reactions with high valency reactants are more likely to occur on defects (more structure-sensitive), as compared to reactions with low valency reactants. Moreover, the reactions on late transition metals are more likely to proceed on defects than those on the early transition metals.

Production and characterisation of polyclonal antibodies to a range of nitroimidazoles.

The nitroimidazoles dimetridazole and ronidazole are metabolised to hydroxydimetridazole, while metronidazole is metabolised to hydroxymetronidazole. To screen for a large number of samples by immunoassay for the presence of this family of drugs and metabolites, it was necessary to produce an antibody with broad-spectrum recognition. Metronidazole and hydroxydimetridazole were selected as antigens as they could be coupled to large (immunogenic) carrier proteins at two different positions of the general nitroimidazole structure. The resulting conjugates were used to immunise rabbits, sheep and goats. Seventeen out of thirty-nine animals immunised produced a detectable antibody titre and these antibodies were consequently characterised as regards sensitivity and cross-reactivity.

The panel of antisera produced exhibited IC50 ranging from 1.26 to 73.76 ng ml-1 using a competitive ELISA assay. Cross-reactivity studies showed that sera from several animals were capable of significant binding of six of the seven nitroimidazole compounds tested.

Structural and chemical embrittlement of grain boundaries by impurities: A general theory and first-principles calculations for copper

First-principles calculations of the Sigma 5(310)[001] symmetric tilt grain boundary in Cu with Bi, Na, and Ag substitutional impurities provide evidence that in the phenomenon of Bi embrittlement of Cu grain boundaries electronic effects do not play a major role; on the contrary, the embrittlement is mostly a structural or "size" effect. Na is predicted to be nearly as good an embrittler as Bi, whereas Ag does not embrittle the boundary in agreement with experiment. While we reject the prevailing view that "electronic" effects (i.e., charge transfer) are responsible for embrittlement, we do not exclude the role of chemistry. However, numerical results show a striking equivalence between the alkali metal Na and the semimetal Bi, small differences being accounted for by their contrasting "size" and "softness" (defined here). In order to separate structural and chemical effects unambiguously if not uniquely, we model the embrittlement process by taking the system of grain boundary and free surfaces through a sequence of precisely defined gedanken processes; each of these representing a putative mechanism. We thereby identify three mechanisms of embrittlement by substitutional impurities, two of which survive in the case of embrittlement or cohesion enhancement by interstitials. Two of the three are purely structural and the third contains both structural and chemical elements that by their very nature cannot be further unraveled. We are able to take the systems we study through each of these stages by explicit computer simulations and assess the contribution of each to the net reduction in intergranular cohesion. The conclusion we reach is that embrittlement by both Bi and Na is almost exclusively structural in origin; that is, the embrittlement is a size effect.

Resonance Raman and lifetime studies on regioselectively deuteriated ruthenium (II) polypyridyl complexes

Two series of ruthenium(II) polypyridyl complexes [Ru(bipy)2(phpytr)]+ and [Ru(bipy)2(phpztr)]+ (where Hphpytr = 2-(5-phenyl-1H-[1,2,4]triazol-3-yl)-pyridine and Hphpztr = 2-(5-phenyl-1H-[1,2,4]triazol-3-yl)-pyrazine) are examined by electrochemistry, UV/Vis, emission, resonance Raman, transient resonance Raman and transient absorption spectroscopy, in order to obtain a more comprehensive understanding of their excited state electronic properties. The interpretation of the results obtained is facilitated by the availability of several isotopologues of each of the complexes examined. For the pyridine-1,2,4-triazolato based complex the lowest emissive excited state is exclusively bipy based, however, for the pyrazine based complexes excited state localisation on particular ligands shows considerable solvent and pH dependency.

General trends in the barriers of catalytic reactions on transition metal surfaces

A catalyst preparation by design is one of the ultimate goals in chemistry. The first step towards this goal is to understand the origin of reaction barriers. In this study, we have investigated several catalytic reactions on some transition metal surfaces, using density functional theory. All the reaction barriers have been determined. By detailed analyses we obtain some insight into the reaction barrier. Each barrier is related to (i) the potential energy surface of reactants on the surface, (ii) the total chemisorption energy of reactants, and (iii) the metal d orbital occupancy and the reactant valency. (C) 2001 American Institute of Physics.

Chemoenzymatic synthesis of the trans-dihydrodiol isomers of monosubstituted benzenes via anti-benzene dioxides

Enantiopure cis-2,3-dihydrodiols, available from dioxygenase-catalysed cis-dihydroxylation of monosubstituted benzene substrates, have been used as synthetic precursors of the corresponding trans-3,4-dihydrodiols. The six-step chemoenzymatic route from cis-dihydrodiol precursors, involving acetonide, tetraol, dibromodiacetate and diepoxide intermediates, and substitution of vinyl bromide and iodide atoms, has been used in the synthesis of ten trans-dihydrododiol derivatives of substituted benzenes. The general applicability of the method has been demonstrated by its use in the synthesis of both enantiomers of the trans-1,2- and 3,4-dihydrodiol derivatives of toluene.

Enzymatic and chemoenzymatic synthesis of arene trans-dihydrodiols

Several potential approaches to the enzyme-catalysed synthesis of arene trans-diols have been examined including epoxidation/hydrolysis, bis-benzylic hydroxylation, cis-dihydroxylation/alcohol dehydrogenation/ketone reduction, cisdihydroxylation/cis-trans isomerisation. and multi-enzyme synthesis of trans-dihydrodiol secondary metabolites from primary metabolites. The lack of general applicability of these enzymatic methods has led to the development of several chemoenzymatic routes for the synthesis of a series of trans-dihydrodiols from the readily available cis-dihydrodiol precursors. Partial hydrogenation of cis-dihydrodiol metabolites to yield the corresponding cis-tetrahydrodiols followed by a regioselective Mitsunobu inversion process gave trans-tetrahydrodiols that were in turn converted to trans-dihydrodiols. The formation of anti-benzene dioxides or iron tricarbonyl complexes from the corresponding cis-dihydrodiol precursors provided shorter and more convenient chemoenzymatic routes to trans-dihydrodiols. The application of cis-dihydrodiol metabolites of polycyclic azaarenes in the synthesis of the corresponding arene oxides followed by chemical hydrolysis provides a convenient route to trans-dihydrodiols. (C) 2002 Elsevier Science B.V. All rights reserved.

The stereoselective coordination chemistry of the helicating ligand N,N '-bis(-2,2 '-dipyridyl-5-yl)carbonyl-(S/R,S/R)-1,2-diphenylethylenediamine

Enantiomerically pure N,N'-bis(-2,2'-dipyridyl-5-yl)carbonyl-(S/R,S/R)-1,2-diphenylethylenediamine has been synthesised by linking two 2,2'-bipyridine units by (R,R)- and (S,S)-1,2-diphenylethylenediamine. The ligands possess a hindered rotation between the bipyridine chromophores, which are held together by intramolecular hydrogen bonds. ES mass spectroscopy confirmed that reaction with Fe(II), Co(III) and Cd(II) afforded dinuclear complexes. CD spectroscopy implied that enantiopure ligands conferred helicity to the metals centre giving a dominant triple helicate diastereoisomer (with the RR isomer giving a P helicate). H-1 NMR spectroscopy of the cadmium complex confirmed the presence of a single diastereoisomer. (C) 2003 Elsevier B.V. All rights reserved.

Increased Dispersion of Supported Gold during Methanol Carbonylation Conditions

The active site in supported gold catalysts for the carbonylation of methanol has been identified as dimers/trimers of gold which are formed from large gold particles >10 nm in diameter. Methyl iodide was found to be critical for this dispersion process and to maintain the catalyst in the active form. This study also shows that it may be possible to redisperse gold catalysts, in general, after reaction.

The role of advanced glycation end products in retinal ageing and disease

The retina is exposed to a lifetime of potentially damaging environmental and physiological factors that make the component cells exquisitely sensitive to age-related processes. Retinal ageing is complex and a raft of abnormalities can accumulate in all layers of the retina. Some of this pathology serves as a sinister preamble to serious conditions such as age-related macular degeneration (AMD) which remains the leading cause of irreversible blindness in the Western world.

The formation of advanced glycation end products (AGEs) is a natural function of ageing but accumulation of these adducts also represents a key pathophysiological event in a range of important human diseases. AGEs act as mediators of neurodegeneration, induce irreversible changes in the extracellular matrix, vascular dysfunction and pro-inflammatory signalling. Since many cells and tissues of the eye are profoundly influenced by such processes, it is fitting that advanced glycation is now receiving considerable attention as a possible pathogenic factor in visual disorders.

This review presents the current evidence for a pathogenic role for AGEs and activation of the receptor for AGEs (RAGE) in initiation and progression of retinal disease. It draws upon the clinical and experimental literature and highlights the opportunities for further research that would definitively establish these adducts as important instigators of retinal disease. The therapeutic potential for novel agents that can ameliorate AGE formation of attenuate RAGE signalling in the retina is also discussed.

Evaluation of glycated glucagon-like peptide-1(7-36)amide in intestinal tissue of normal and diabetic animal models

Glucagon-like peptide-1(7-36)amide (tGLP-1) is an important insulin-releasing hormone of the enteroinsular axis which is secreted by endocrine L-cells of the small intestine following nutrient ingestion. The present study has evaluated tGLP-1 in the intestines of normal and diabetic animal models and estimated the proportion present in glycated form. Total immunoreactive tGLP-1 levels in the intestines of hyperglycaemic hydrocortisone-treated rats, streptozotocin-treated mice and ob/ob mice were similar to age-matched controls. Affinity chromatographic separation of glycated and non-glycated proteins in intestinal extracts followed by radioimmunoassay using a fully crossreacting anti-serum demonstrated the presence of glycated tGLP-1 within the intestinal extracts of all control animals (approximately 19%., of total tGLP-1 content). Chemically induced and spontaneous animal models of diabetes were found to possess significantly greater levels of glycated tGLP-1 than controls, corresponding to between 24-71% of the total content. These observations suggest that glycated tGLP-1 may be of physiological significance given that such N-terminal modification confers resistance to DPP IV inactivation and degradation, extending the very short half-life (