The Averaged Dynamics of the Hydrogen Atom in Crossed Electric and Magnetic Fields as a Perturbed Kepler Problem

We treat the classical dynamics of the hydrogen atom in perpendicular electric and magnetic fields as a celestial mechanics problem. By expressing the Hamiltonian in appropriate action–angle variables, we separate the different time scales of the motion. The method of averaging then allows us to reduce the system to two degrees of freedom, and to classify the most important periodic orbits.     

Generalization of Saint-Venant's principle to micropolar continua

The mathematical formulation and proof of Saint-Venant's principle as given by Toupin for non-polar solids is generalized to the case of micropolar elasticity. On one end of a micropolar cylinder of arbitrary length and cross-section we apply a system of self-equilibrated stresses and couple stresses. We first prove that the norms of the stress and couple stress tensors are bounded by the energy density. By means of Rayleigh's principle for the lowest natural eigenfrequency for a slice of the cylinder we then prove that the energy, stored in the cylinder beyond a certain distance from the loaded end, has an exponential decrease with this distance, thus establishing Saint-Venant's principle for the system.     

Test of the generalized standard addition method and linearization algorithms for the direct,simultaneous, multi-element analysis of solid samples by electrothermal atomic absorption spectrometry

The application of a multi-element electrothermal atomic absorption spectrometry (ETAAS) instrument with line sources and Zeeman-effect background correction to the direct, simultaneous determination of Ag, Cd, Cr and Pb in solid reference materials is described. Due to the occurrence of interference effects, the generalized standard addition method (GSAM) was required for calibration purposes. In order to extend the dynamic range for analytes present at high concentrations, linearization algorithms were tested. The combination of the GSAM and extension of the linear range, when necessary, generally yielded acceptable analytical results for the suite of elements studied, and may provide a useful approach to calibration in the direct analysis of solid samples by multi-element ETAAS. However, while linearization yielded good results for Ag and Cd in solid reference materials, it remains to be demonstrated that acceptable performance can be obtained for other elements in real samples.     

Creation of a synthetically useful lipase with higher than wild-type enantioselectivity and maintained catalytic activity

[reaction: see text] We have found that two Geotrichum candidum lipase isozymes have remarkably different abilities to differentiate between enantiomers of ethyl 2-methyldecanoate. By rational recombination of selected portions of the two isozymes, we have created a novel lipase with an enantioselectivity superior to that of the best wild-type parent isozyme. Site-directed mutagenesis identified two key amino acid residues responsible for the improved enantioselectivity without compromised total activity of the reengineered enzyme.     

Carbon-carbon bonds by hydrolytic enzymes

Enzymes are efficient catalysts in synthetic chemistry, and their catalytic activity with unnatural substrates in organic reaction media is an area attracting much attention. Protein engineering has opened the possibility to change the reaction specificity of enzymes and allow for new reactions to take place in their active sites. We have used this strategy on the well-studied active-site scaffold offered by the serine hydrolase Candida antarctica lipase B (CALB, EC 3.1.1.3) to achieve catalytic activity for aldol reactions. The catalytic reaction was studied in detail by means of quantum chemical calculations in model systems. The predictions from the quantum chemical calculations were then challenged by experiments. Consequently, Ser105 in CALB was targeted by site-directed mutagenesis to create enzyme variants lacking the nucleophilic feature of the active site. The experiments clearly showed an increased reaction rate when the aldol reaction was catalyzed by the mutant enzymes as compared to the wild-type lipase. We expect that the new catalytic activity, harbored in the stable protein scaffold of the lipase, will allow aldol additions of substrates, which cannot be reached by traditional aldolases.     

Effect of pectin extraction method on properties of cellulose nanofibers isolated from sugar beet pulp

In this study, the effect of pectin extraction method on the properties of cellulose nanofibers (CNFs) isolated from sugar beet pulp (SBP) was studied. Pectin was extracted by the industrially practiced method by sulfuric acid hydrolysis or by enzymatic hydrolysis using a cellulase/xylanase enzymes mixture. The CNFs were then isolated by high-pressure homogenization and investigated in terms of their chemical composition, crystallinity, size, degree of polymerization, and re-dispersion in water after freeze-drying. The mechanical properties and surface characteristics of CNF films were also studied. The results showed that fibrillation of the de-pectinated SBP was more efficient for the acid hydrolyzed SBP. CNFs from the acid-hydrolyzed SBP had a slightly wider diameter, higher crystallinity, viscosity, and α-cellulose content but a lower degree of polymerization than CNFs from the enzyme-hydrolyzed SBP. Owing to the presence of more residual hemicelluloses in the CNFs from the enzyme-hydrolyzed SBP, the CNFs had higher re-dispersion ability in water. CNF films from enzyme-hydrolyzed SBP displayed slightly better mechanical properties and higher water contact angle than acid-hydrolyzed CNF films.

Graphic abstract

     

Ruthenium-manganese complexes for artificial photosynthesis: factors controlling intramolecular electron transfer and excited-state quenching reactions

Continuing our work toward a system mimicking the electron-transfer steps from manganese to P(680)(+) in photosystem II (PS II), we report a series of ruthenium(II)-manganese(II) complexes that display intramolecular electron transfer from manganese(II) to photooxidized ruthenium(III). The electron-transfer rate constant (k(ET)) values span a large range, 1 x 10(5)-2 x 10(7) s(-1), and we have investigated different factors that are responsible for the variation. The reorganization energies determined experimentally (lambda = 1.5-2.0 eV) are larger than expected for solvent reorganization in complexes of similar size in polar solvents (typically lambda approximately 1.0 eV). This result indicates that the inner reorganization energy is relatively large and, consequently, that at moderate driving force values manganese complexes are not fast donors. Both the type of manganese ligand and the link between the two metals are shown to be of great importance to the electron-transfer rate. In contrast, we show that the quenching of the excited state of the ruthenium(II) moiety by manganese(II) in this series of complexes mainly depends on the distance between the metals. However, by synthetically modifying the sensitizer so that the lowest metal-to-ligand charge transfer state was localized on the nonbridging ruthenium(II) ligands, we could reduce the quenching rate constant in one complex by a factor of 700 without changing the bridging ligand. Still, the manganese(II)-ruthenium(III) electron-transfer rate constant was not reduced. Consequently, the modification resulted in a complex with very favorable properties.