Oxidation of cyclohexane by a high-valent iron bispidine complex: a combined experimental and computational mechanistic study

Experimental data suggest that there are various competing pathways for the catalytic and stoichiometric oxygenation of cyclohexane, assisted by iron-bispidine complexes and using various oxidants (H(2)O(2), O(2), PhIO). Density functional theory calculations indicate that both Fe(IV)=O and Fe(V)=O species are accessible and efficiently transfer their oxygen atoms to cyclohexane. The reactivities of the two isomers each and the two possible spin states for the Fe(IV)=O and Fe(V)=O species are sufficiently different to allow an interpretation of the experimental data.     

Analysis of carbohydrates by anion exchange chromatography and mass spectrometry

A versatile liquid chromatographic platform has been developed for analysing underivatized carbohydrates using high performance anion exchange chromatography (HPAEC) followed by an inert PEEK splitter that splits the effluent to the integrated pulsed amperometric detector (IPAD) and to an on-line single quadrupole mass spectrometer (MS). Common eluents for HPAEC such as sodium hydroxide and sodium acetate are beneficial for the amperometric detection but not compatible with electrospray ionisation (ESI). Therefore a membrane-desalting device was installed after the splitter and prior to the ESI interface converting sodium hydroxide into water and sodium acetate into acetic acid. To enhance the sensitivity for the MS detection, 0.5 mmol/l lithium chloride was added after the membrane desalter to form lithium adducts of the carbohydrates. To compare sensitivity of IPAD and MS detection glucose, fructose, and sucrose were used as analytes. A calibration with external standards from 2.5 to 1000 pmole was performed showing a linear range over three orders of magnitude. Minimum detection limits (MDL) with IPAD were determined at 5 pmole levels for glucose to be 0.12 pmole, fructose 0.22 pmole and sucrose 0.11 pmole. With MS detection in the selected ion mode (SIM) the lithium adducts of the carbohydrates were detected obtaining MDL's for glucose of 1.49 pmole, fructose 1.19 pmole, and sucrose 0.36 pmole showing that under these conditions IPAD is 3-10 times more sensitive for those carbohydrates. The applicability of the method was demonstrated analysing carbohydrates in real world samples such as chicory inulin where polyfructans up to a molecular mass of 7000 g/mol were detected as quadrupoly charged lithium adducts. Furthermore mono-, di-, tri-, and oligosaccharides were detected in chicory coffee, honey and beer samples.     

Rationalizing perhydrolase activity of aryl-esterase and subtilisin Carlsberg mutants by molecular dynamics simulations of the second tetrahedral intermediate state

The perhydrolysis reaction in hydrolases is an important example of catalytic promiscuity and has many potential industrial applications. The mechanisms of perhydrolase activity of a subtilisin Carlsberg mutant and of an aryl-esterase mutant have been investigated using classical molecular dynamics simulations of the second tetrahedral intermediate (TI) state. The simulations demonstrated that hydrogen bonding between the second TI of the perhydrolysis reaction is possible in the mutants but not wild type. The stabilization by hydrogen bonds was specific for the perhydrolysis intermediate and either no hydrogen bonding or only weakened hydrogen bonding to the second TI state of the hydrolysis reaction was observed. Furthermore, a significant hindrance to the formation of the catalytically important hydrogen bond between His64 and Ser221 in the catalytic triad by competing hydrogen bonds was found for the subtilisin mutant but not wild type enzyme in case of the hydrolysis intermediate. The opposite was observed in case of the perhydrolysis intermediate. The result offers a qualitative explanation for the overall reduced hydrolysis activity of the subtilisin mutant. In addition, the simulations also explain qualitatively the perhydrolysis activity of the enzyme variants and may be helpful for designing enzyme mutants with further improved perhydrolysis activity.     

Thermal expansivity of particulate composites: Interlayer versus molecular model

We continue the comparison of the results of an interlayer model, based on the theory of elastic continua, and a molecular model, derived from a theory of mixtures, previously presented in terms of bulk moduli K. We now derive expressions for the dependence of the thermal expansivity _c on the volume fraction _f of the filler, at low and elevated values of _f . Correspondencies between the characteristic parameters, viz. adhesion and repulsion ratios on the one hand, and interlayer content and thermal properties of matrix, filler, and layer, on the other, are examined. Since in the molecular theory both andK are derived from an equation of state, the identical set of parameters determines both functions and suggests correlations between them.     

Large minimal realizers of a partial order II

The rank (resp. dimension) of a poset P is the cardinality of a largest (resp. smallest) set of linear orders such that its intersection is P and no proper subset has intersection P. Dimension has been studied extensively. Rank was introduced recently by Maurer and Rabinovitch in [4], where the rank of antichains was determined. In this paper we develop a general theory of rank. The main result, loosely stated, is that to each poset P there corresponds a class of graphs with easily described properties, and that the rank of P is the maximum number of edges in a graph in this class.     

On ϵ productions for terminals in E0L forms

When working with E0L forms, equality of languages and of language families has traditionally been considered modulo the empty word ? to avoid trivial but cumbersome distinctions of cases. Despite this fact, the role of ?-productions is often significant in L form theory particular, it has previously been shown that vomplete forms must have ?-productions We strengthen this result by proving that vomplete forms must even have ?-productions for nonterminals. Based on this theorem we establish that there are E0L forms with the property that any form equivalent E0L form must generate the empty word. Thus, despite the fact that we define equality of languages modulo ?, we are still unable to ignore the presence of the empty word in languages when studying L forms. The proof of our main theorem is based on a number of lemmas which allow to show that certain E0L forms are not form equivalent. In view of the difficulty of proving or disproving form equivalence of arbitrary E0L forms (conjectured to be undecidable) these lemmas may well be of independent interest.