Catalytic properties of heteropolyacids supported on MCM-41 mesoporous silica for hydrocarbon cracting reactions

It is known that MCM-41 structures have very weak acid sites because of the lack of the bridging hydroxyl groups present in zeolites. Strong acidity however is required for the potential use of these materials in some specific applications such as: cracking and hydrotreating of heavy residue molecules, cracking of waste plastic, etc. The acidity enhancement of the MCM-41 materials was assessed using the n-hexane and polyethylene cracking reactions. MCM-41 samples were impregnated using heteropolyacid (HPA) such as tungestophospheric acid. The catalyst samples were characterized also by x-ray diffraction and benzene adsorption.

Nucleophilic substitution reactions of pyranose polytosylates

The 2,3,4-tri-toluenesulfonate ester derivatives of the methyl pyranosides of L-arabinose, D-ribose, D-lyxose, and D-xylose have been prepared, and their substitution reactions with various nucleophiles have been examined. For arabinose, xylose, and ribose, highly regioselective monosubstitutions were observed with benzoate, nitrite, and azide anions. These reactions have led to short and simple routes from D-xylose to L-arabinose derivatives, from L-arabinose to D-xylose derivatives, and from D-ribose to L-lyxose derivatives. The tritosylate derived from methyl alpha-D-lyxopyranoside was unreactive toward nucleophilic substitution reactions, giving instead a dihydropyran product arising from an initial E2 elimination reaction of the 2-tosylate.

A potential role for isothermal calorimetry in studies of the effects of thermodynamic non-ideality in enzyme-catalyzed reactions

Attention is drawn to the feasibility of using isothermal calorimetry for the characterization of enzyme reactions under conditions bearing greater relevance to the crowded biological environment, where kinetic parameters are likely to differ significantly from those obtained by classical enzyme kinetic studies in dilute solution. An outline of the application of isothermal calorimetry to the determination of enzyme kinetic parameters is followed by considerations of the nature and consequences of crowding effects in enzyme catalysis. Some of those effects of thermodynamic non-ideality are then illustrated by means of experimental results from calorimetric studies of the effect of molecular crowding on the kinetics of catalysis by rabbit muscle pyruvate kinase. This review concludes with a discussion of the potential of isothermal calorimetry for the experimental determination of kinetic parameters for enzymes either in biological environments or at least in media that should provide reasonable approximations of the crowded conditions encountered in vivo. Copyright (C) 2004 John Wiley Sons, Ltd.

ASCIA guidelines for prevention of food anaphylactic reactions in schools, preschools and child-care centres

These guidelines have been developed by the anaphylaxis working party of the Australasian Society of Clinical Immunology and Allergy to provide advice for minimizing the risk of food-induced anaphylaxis in schools, preschools and child-care centres. The guidelines outline four steps for the prevention of food anaphylactic reactions in children at risk and food policy measures specific to school age and preschool age children.

Transition-state-theory calculations for reactions of O(3P) with halogenated olefins

The potential energy surfaces for the reactions of atomic oxygen in its ground electronic state, O(P-3), with the olefins: CF2=CCl2 and CF2=CF - CF3, have been characterized using ab initio molecular orbital calculations. Geometry optimization and vibrational frequency calculations were performed for reactants, transition states and products at the MP2 and QCISD levels of theory using the 6-31G(d) basis set. This database was then used to calculate the rate constants by means of Transition-State-Theory. To obtain a better reference and to test the reliability of the activation barriers we have also carried out computations using the CCSD(T)(fc)/6-311Gdagger, MP4(SDQ)(fc)/CBSB4 and MP2(fc)/CBSB3 single point energy calculations at both of the above levels of theory, as well as with the composite CBS-RAD procedure ( P. M. Mayer, C. J. Parkinson, D. M. Smith and L. Radom, J. Chem. Phys., 1998, 108, 604) and a modi. cation of this approach, called: CBS-RAD( MP2, MP2). It was found that the kinetic parameters obtained in this work particularly with the CBS-RAD ( MP2, MP2) procedure are in reasonable agreement with the experimental values. For both reactions it is found that the channels leading to the olefin double-bond addition predominates with respect to any other reaction pathway. However, on account of the different substituents in the alkenes we have located, at all levels of theory, two transition states for each reaction. Moreover, we have found that, for the reactions studied, a correlation exists between the activation energies and the electronic structure of the transition states which can explain the influence of the substituent effect on the reactivity of the halo-olefins.

Calibration of the channel that determines the omega-hydroxylation regiospecificity of cytochrome P4504A1 - Catalytic oxidation of 12-halododecanoic acids

The fatty acid omega-hydroxylation regiospecificity of CYP4 enzymes may result from presentation of the terminal carbon to the oxidizing species via a narrow channel that restricts access to the other carbon atoms. To test this hypothesis, the oxidation of 12-iodo-, 12-bromo-, and 12-chlorododecanoic acids by recombinant CYP4A1 has been examined. Although all three 12-halododecanoic acids bind to CYP4A1 with similar dissociation constants, the 12-chloro and 12-bromo fatty acids are oxidized to 12-hydroxydodecanoic acid and 12-oxododecanoic acid, whereas the 12-iodo analogue is very poorly oxidized. Incubations in (H2O)-O-18 show that the 12-hydroxydodecanoic acid oxygen derives from water, whereas that in the aldehyde derives from O-2. The alcohol thus arises from oxidation of the halide to an oxohalonium species that is hydrolyzed by water, whereas the aldehyde arises by a conventional carbon hydroxylation-elimination mechanism. No irreversible inactivation of CYP4A1 is observed during 12-halododecanoic acid oxidation. Control experiments show that CYP2E1, which has an omega-1 regiospecificity, primarily oxidizes 12-halododecanoic acids to the omega-aldehyde rather than alcohol product. Incubation of CYP4A1 with 12,12-[H-2](2)-12-chlorododecanoic acid causes a 2-3-fold increase in halogen versus carbon oxidation. The fact that the order of substrate oxidation (Br > Cl >> I) approximates the inverse of the intrinsic oxidizability of the halogen atoms is consistent with presentation of the halide terminus via a channel that accommodates the chloride and bromide but not iodide atoms, which implies an effective channel diameter greater than 3.90 angstrom but smaller than 4.30 angstrom.

Competitive sorption reactions between phosphorus and organic matter in soil: a review

The incorporation of organic matter ( OM) in soils that are able to rapidly sorb applied phosphorus ( P) fertiliser reportedly increases P availability to plants. This effect has commonly been ascribed to competition between the decomposition products of OM and P for soil sorption sites resulting in increased soil solution P concentrations. The evidence for competitive inhibition of P sorption by dissolved organic carbon compounds, derived from the breakdown of OM, includes studies on the competition between P and (i) low molecular weight organic acids (LOAs), (ii) humic and fulvic acids, and (iii) OM leachates in soils with a high P sorption capacity. These studies, however, have often used LOAs at 1 - 100 mM, concentrations much higher than those in soils ( generally < 0.05 mM). The transience of LOAs in biologically active soils further suggests that neither their concentration nor their persistence would have a practical benefit in increasing P phytoavailability. Higher molecular weight compounds such as humic and fulvic acids also competitively inhibit P sorption; however, little consideration has been given to the potential of these compounds to increase the amount of P sorbed through metal - chelate linkages. We suggest that the magnitude of the inhibition of P sorption by the decomposition products of OM leachate is negligible at rates equivalent to those of OM applied in the field. Incubation of OM in soil has also commonly been reported as reducing P sorption in soil. However, we consider that the reported decreases in P sorption ( as measured by P in the soil solution) are not related to competition from the decomposition products of OM breakdown, but are the result of P release from the OM that was not accounted for when calculating the reduction in P sorption.

Exploring the potential of xenobiotic-metabolising enzymes as biocatalysts: Evolving designer catalysts from polyfunctional cytochrome P450 enzymes

1. Biological catalysts have the advantage of being able to catalyse chemical reactions with an often exquisite degree of regio- and stereospecificity in contrast with traditional methods of organic synthesis. 2. The cytochrome P450 enzymes involved in human drug metabolism are ideal starting materials for the development of designer biocatalysts by virtue of their catalytic versatility and extreme substrate diversity. Applications can be envisaged in fine chemical synthesis, such as in the pharmaceutical industry and bioremediation. 3. A variety of techniques of enzyme engineering are currently being applied to P450 enzymes to explore their catalytic potential. Although most studies to date have been performed with bacterial P450s, reports are now emerging of work with mammalian forms of the enzymes. 4. The present minireview will explore the rationale and general techniques for redesigning P450s, review the results obtained to date with xenobiotic-metabolising forms and discuss strategies to overcome some of the logistic problems limiting the full exploitation of these enzymes as industrial-scale biocatalysts.

Intra- vs intermolecular photoinduced electron transfer reactions of a macrocyclic donor-acceptor dyad

The synthesis, structural characterization, and photophysical behavior of a 14-membered tetraazamacrocycle with pendant 4-dimethylaminobenzyl (DMAB) and 9-anthracenylmethyl groups is reported (L-3, 6-((9-anthracenylmethyl)amino)-trans-6,13-dimethyl-13-((4-dimethylaminobenzyl)amino)-1,4,8,11-tetraaza-cyclotetradecane). In its free base form, this compound displays rapid intramolecular photoinduced electron transfer (PET) quenching of the anthracene emission, with both the secondary amines and the DMAB group capable of acting as electron donors. When complexed with Zn(II), the characteristic fluorescence of the anthracene chromophore is restored as the former of these pathways is deactivated by coordination. Importantly, it is shown that the DMAB group, which remains uncoordinated and PET active, acts only very weakly to quench emission, by comparison to the behavior of a model Zn complex lacking the pendant DMAB group, [ZnL2](2+) (Chart 1). By contrast, Stern-Volmer analysis of intermolecular quenching of [ZnL2](2+) by N,N-dimethylaniline (DMA) has shown that this reaction is diffusion limited. Hence, the pivotal role of the bridge in influencing intramolecular PET is highlighted.

Are insect immune suppressors driving cellular uptake reactions?

Many insect parasitoids that deposit their eggs inside immature stages of other insect species inactivate the cellular host defence to protect the growing embryo from encapsulation. Suppression of encapsulation by polydnavirus-encoded immune-suppressors correlates with specific alterations in hemocytes, mainly cytoskeletal rearrangements and actin-cytoskeleton breakdown. We have previously shown that the Cotesia rubecula polydnavirus gene product CrV1 causes immune suppression when injected into the host hemocoel. CrV1 is taken up by hemocytes although no receptors have been found to bind the protein. Instead CrV1 uptake depends on dimer formation, which is required for interacting with lipophorin, suggesting a CrV1-lipophorin complex internalisation by hemocytes. Since treatment of hemocytes with oligomeric lectins and cytochalasin D can mimic the effects of CrV1, we propose that some dimeric and oligomeric adhesion molecules are able to cross-link receptors on the cell surface and depolymerise actin by leverage-mediated clearance reactions in the hemolymph.

The role of leader-member exchanges in mediating the relationship between locus of control and work reactions

The relationship between locus of control, the quality of exchanges between subordinates and leaders (LMX), and a variety of work-related reactions (intrinsic/extrinsic job satisfaction, work-related well-being, and organizational commitment) are examined. It was predicted that people with an internal locus of control develop better quality relations with their manager and this, in turn, results in more favourable work-related reactions. Results from two different samples (N = 404, and N = 51) supported this prediction, and also showed that LMX either fully, or partially, mediated the relationship between locus of control and all the work-related reactions.

A gradient descent algorithm for minimizing amino acid coupling reactions when synthesizing cyclic-peptide libraries

Combinatorial chemistry has become an invaluable tool in medicinal chemistry for the identification of new drug leads. For example, libraries of predetermined sequences and head-to-tail cyclized peptides are routinely synthesized in our laboratory using the IRORI approach. Such libraries are used as molecular toolkits that enable the development of pharmacophores that define activity and specificity at receptor targets. These libraries can be quite large and difficult to handle, due to physical and chemical constraints imposed by their size. Therefore, smaller sub-libraries are often targeted for synthesis. The number of coupling reactions required can be greatly reduced if the peptides having common amino acids are grouped into the same sub-library (batching). This paper describes a schedule optimizer to minimize the number of coupling reactions by rotating and aligning sequences while simultaneously batching. The gradient descent method thereby reduces the number of coupling reactions required for synthesizing cyclic peptide libraries. We show that the algorithm results in a 75% reduction in the number of coupling reactions for a typical cyclic peptide library.

Adolescents' reactions to universal and indicated prevention programs for depression: Perceived stigma and consumer satisfaction

There is a common view that one of the major considerations in selecting between universal and indicated interventions is the marked stigma produced by the latter. However, to date there has been no empirical examination of this assumption. The current study examined reported stigma and program satisfaction following two school-based interventions aimed at preventing depression in 532 middle adolescents. The interventions were conducted either across entire classes by classroom teachers (universal delivery) or in small high risk groups by mental health professionals (indicated delivery). The indicated delivery was associated with significantly greater levels of perceived stigma, but effect sizes were small and neither program was associated with marked stigma in absolute terms. Perceived stigma was more strongly associated with aspects of the individual including being male and showing greater externalizing symptomatology. In contrast, the indicated program was evaluated more positively by both participants and program leaders and effect sizes for these measures of satisfaction were moderate to large. The results point to the need for further empirical evaluation of both perceived stigma and program satisfaction in providing balanced considerations of the value of indicated and universal programs.

Cognitive asymmetry in employee emotional reactions to leadership behaviors

This article is predicated on the idea that leaders shape workplace affective events. Based on Affective Events Theory (AET), I argue that leaders are sources of employee positive and negative emotions at work. Certain leader behaviors displayed during interactions with their employees are the sources of these affective events. The second theoretical underpinning of the article is the Asymmetry Effect of emotion. Consistent with this theory, employees are more likely to recall negative incidents than positive incidents. In a qualitative study, evidence that these processes exist in the workplace was found. Leader behaviors were sources of positive or negative emotional responses in employees; employees recalled more negative incidents than positive incidents, and they recalled them more intensely and in more detail than positive incidents. Consequently, leaders may need to exercise their emotional intelligence to generate emotional uplifts to overcome the hassles in the workplace that employees seem to remember so vividly. (c) 2006 Elsevier Inc. All rights reserved.