Characterization of partially transesterified poly(beta-hydroxyalkanoate)s using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS) was used for the characterization of a partially transesterified poly(beta-hydroxyalkanoate), PHA, polymer produced by the bacterial strain Alcaligenes eutrophus using saponified vegetable oils as the sole carbon sources. The transesterification was carried out separately under acidic and basic conditions to obtain PHA oligomers weighing less than 10 kDa. The intact oligomers were detected in their cationized [M + Na](+) and [M + K](+) forms by MALDI-TOFMS. A composition analysis, using the MALDI-TOF spectra, indicate that the oligomers obtained via acid catalysis were terminated with a methyl 3-hydroxybutyrate end group, and those obtained by base catalysis had a methyl crotonate (olefinic) termination. In addition to HB (hydroxy butyrate), the oligomers were found to contain a small percentage of HV (hydroxy valerate). This was independently confirmed using gas chromatography/mass spectrometry (GC/MS). In comparison, the analysis of a commercial PHA polymer, transesterified under identical conditions, only showed the presence of HB, i.e. a pure PHB homopolymer. Copyright 1999 John Wiley & Sons, Ltd.     

Quantification of substituent and ligand size by the use of solid angles

A new, generalized, method for measuring the steric size of ligands and substituents has been developed. The method is based on the solid angle concept, the solid angle being generated by the pairwise addition of spheres (atoms). The problem of overlapping spheres (atoms) has been solved analytically and the algorithm provided permits evaluation of the steric space occupied by any combination of spheres (atoms). A range of solid angles for commonly encountered ligands and substituents has been determined and compared to literature values for linear cone angles. © John Wiley & Sons, Inc.     

Structure and Reactivity of Al−O(H)−Al Moieties in Siloxide Frameworks: Solution and Gas‐Phase Model Studies

Even though aluminas and aluminosilicates have found widespread application, a consistent molecular understanding of their surface heterogeneity and the behavior of defects resulting from hydroxylation/dehydroxylation remains unclear. Here, we study the well‐defined molecular model compound, [Al₃(μ₂‐OH)₃(THF)₃(PhSi(OSiPh₂O)₃)₂], 1 , to gain insight into the acid–base reactivity of cyclic trinuclear Al₃(μ₂‐OH)₃ moieties at the atomic level. We find that, like zeolites, they are sufficiently acidic to catalyze the isomerization of olefins. DFT and gas phase vibrational spectroscopy on solvent‐free and deprotonated 1 show that the six‐membered ring structure of its Al₃(μ₂‐OH)₃ core is unstable with respect to deprotonation of one of its hydroxy groups and rearranges into two edge‐sharing four‐membered rings. This renders Al_IV?O(H)?Al_IV units strong acid sites, and all results together suggest that their acidity is similar to that of zeolitic Si_IV?O(H)?Al_IV groups.     

The role of structural heat exchange and heat loss in the design of efficient silicon micro-combustors

The performance of millimeter-scale combustors intended for miniaturized power and propulsion systems is strongly influenced by heat exchange to and within the combustor structure. Accordingly, a one-dimensional model with full chemistry that includes heat exchange to and within the combustor wall has been developed. It is used to study the effects of axial heat transfer from the post-flame to the pre-flame via wall conduction in a silicon micro-channel combustor with planar symmetry. The simulations show that axial heat transfer widens stability limits, increases the burning rate, and can enable the construction of smaller, higher power density combustors. Axial heat transfer also enhances the benefits of operating at elevated pressures. The simulations also show that heat loss to the environment places a lower bound on the combustor volume. Maximum power density combustor configurations are identified under adiabatic and non-adiabatic conditions. The maximum power density tends to increase with increasing pressure while the micro-channel length and height associated with the maximum tend to decrease.     

Pattern formation in a 2D simple chemical system with general orders of autocatalysis and decay

In this paper, we investigate pattern formation in a coupledsystem of reaction–diffusion equations in two spatialdimensions. These equations arise as a model of isothermal chemicalautocatalysis with termination in which the orders of autocatalysisand termination, m and n, respectively, are such that 1 <n < m. We build on the preliminary work by Leach & Wei(2003, Physica D, 180, 185–209) for this coupled systemin one spatial dimension, by presenting rigorous stability analysisand detailed numerical simulations for the coupled system intwo spatial dimensions. We demonstrate that spotty patternsare observed over a wide parameter range.     

An efficient method for synthesising unsymmetrical silaketals: substrates for ring-closing, including macrocycle-closing, metathesis

Diisopropylsilyl ethers were activated with N-bromosuccinimide, and reacted with a fluorous-tagged alcohol, to yield tethered substrates for ring-closing metathesis reactions.     

Conformational exchange in pimonidazole--a hypoxia marker

Pimonidazole is one of a series of nitroimidazole compounds that is widely used as a marker for qualitative and quantitative assessment of tumour hypoxia. We have observed a novel dynamic conformational exchange process in this molecule in aqueous solution. By a combination of 1H, 13C, two-dimensional 1H-1H EXchange SpectroscopY (EXSY) and spectral simulation, we unambiguously attribute the conformational exchange process to flipping of the six-membered heterocyclic ring.