Substitution reactions with [ReBr(2)(CO)(2)(NCCH(3))(2)](-): a convenient route to complexes with the cis-[Re(CO)(2)](+) core

Water- and air-stable complexes comprising the cis-[Re(CO)(2)](+) core can be synthesized from the (Et(4)N)[ReBr(2)(NCCH(3))(2)(CO)(2)] precursor . Complex showed distinctly different chemical and electronic behaviour compared to [ReBr(3)(CO)(3)](2-). Substituting the two bromides in with imidazole-like ligands or alpha,alpha'-diimines gave new complexes with potential applications in bioinorganic chemistry and photochemistry. The two acetonitrile ligands are very stably bound and could not be replaced. Under CO pressure, the uncommon complex mer-[ReBr(NCCH(3))(2)(CO)(3)] was formed from . The reaction of with the tetradentate ligand bis(2-pyridylmethyl)glycine (BPG) finally induced a four fold substitution at the metal center to form a [Re(CO)(2)(L(4))](+)-type complex.     

Sharp bounds for the valence of certain harmonic polynomials

In Khavinson and Swiatek (2002) it was proved that harmonic polynomials , where is a holomorphic polynomial of degree , have at most complex zeros. We show that this bound is sharp for all by proving a conjecture of Sarason and Crofoot about the existence of certain extremal polynomials . We also count the number of equivalence classes of these polynomials.

     

All the disorder mechanisms in the 13:58 phases come together. Out of the modulated confusion rises the remarkable phase Ce12.60Cd58.68(2)

The compound Ce12.60Cd58.68(2) is a metrically commensurate representative of the incommensurately modulated phase Ce13Cd57+delta. It is most likely a lock-in phase. The structure, which was solved using seeding of the modulation from those positions most affected as well as direct solution by charge flipping, represents a rare case of ordering in a family of structures where disorder is the rule. The disorder mechanisms, known from other RE13Zn/Cd58 phases, order in this phase to give rise to a remarkable interplay between interstitials and vacancies. The compound crystallizes in the super space group Ccmm(alpha00)00s (standard setting Amma(00gamma)s00) with cell parameters a=27.2789(14) A, b=15.7592(1) A, and c=15.5816(4) A and a q vector close to 2/3.     

A Reactive Molecular Dynamics Study of Chlorinated Organic Compounds. Part II: A ChemTraYzer Study of Chlorinated Dibenzofuran Formation and Decomposition Processes

In our two-paper series, we first present the development of ReaxFF CHOCl parameters using the recently published ParAMS parametrization tool. In this second part, we update the reactive Molecular Dynamics – Quantum Mechanics coupling scheme ChemTraYzer and combine it with our new ReaxFF parameters from Part I to study formation and decomposition processes of chlorinated dibenzofurans. We introduce a self-learning method for recovering failed transition-state searches that improves the overall ChemTraYzer transition-state search success rate by 10 percentage points to a total of 48 %. With ChemTraYzer, we automatically find and quantify more than 500 reactions using transition state theory and DFT. Among the discovered chlorinated dibenzofuran reactions are numerous reactions that are new to the literature. In three case studies, we discuss the set of reactions that are most relevant to the dibenzofuran literature: (i) bimolecular reactions of the chlorinated-dibenzofuran precursors phenoxy radical and 1,3,5-trichlorobenzene, (ii) dibenzofuran chlorination and pyrolysis, and (iii) oxidation of chlorinated dibenzofurans.     

Practical use of robust GCV and modified GCV for spline smoothing

Generalized cross-validation (GCV) is a popular parameter selection criterion for spline smoothing of noisy data, but it sometimes yields a severely undersmoothed estimate, especially if the sample size is small. Robust GCV (RGCV) and modified GCV are stable extensions of GCV, with the degree of stabilization depending on a parameter \(\gamma \in (0,1)\) for RGCV and on a parameter \(\rho >1\) for modified GCV. While there are favorable asymptotic results about the performance of RGCV and modified GCV, little is known for finite samples. In a large simulation study with cubic splines, we investigate the behavior of the optimal values of \(\gamma \) and \(\rho \), and identify simple practical rules to choose them that are close to optimal. With these rules, both RGCV and modified GCV perform significantly better than GCV. The performance is defined in terms of the Sobolev error, which is shown by example to be more consistent with a visual assessment of the fit than the prediction error (average squared error). The results are consistent with known asymptotic results.     

Backbone motions of free and pheromone-bound major urinary protein I studied by molecular dynamics simulation

Molecular motions of free and pheromone-bound mouse major urinary protein I, previously investigated by NMR relaxation, were simulated in 30 ns molecular dynamics (MD) runs. The backbone flexibility was described in terms of order parameters and correlation times, commonly used in the NMR relaxation analysis. Special attention was paid to the effect of conformational changes on the nanosecond time scale. Time-dependent order parameters were determined in order to separate motions occurring on different time scales. As an alternative approach, slow conformational changes were identified from the backbone torsion angle variances, and "conformationally filtered" order parameters were calculated for well-defined conformation states. A comparison of the data obtained for the free and pheromone-bound protein showed that some residues are more rigid in the bound form, but a larger portion of the protein becomes more flexible upon the pheromone binding. This finding is in general agreement with the NMR results. The higher flexibility observed on the fast (fs-ps) time scale was typically observed for the residues exhibiting higher conformational freedom on the ns time scale. An inspection of the hydrogen bond network provided a structural explanation for the flexibility differences between the free and pheromone-bound proteins in the simulations.     

Role of aromatic side chains in the folding and thermodynamic stability of integral membrane proteins

Aromatic residues are frequently found in helical and beta-barrel integral membrane proteins enriched at the membrane-water interface. Although the importance of these residues in membrane protein folding has been rationalized by thermodynamic partition measurements using peptide model systems, their contribution to the stability of bona fide membrane proteins has never been demonstrated. Here, we have investigated the contribution of interfacial aromatic residues to the thermodynamic stability of the beta-barrel outer membrane protein OmpA from Escherichia coli in lipid bilayers by performing extensive mutagenesis and equilibrium folding experiments. Isolated interfacial tryptophanes contribute -2.0 kcal/mol, isolated interfacial tyrosines contribute -2.6 kcal/mol, and isolated interfacial phenylalanines contribute -1.0 kcal/mol to the stability of this protein. These values agree well with the prediction from the Wimley-White interfacial hydrophobicity scale, except for tyrosine residues, which contribute more than has been expected from the peptide models. Double mutant cycle analysis reveals that interactions between aromatic side chains become significant when their centroids are separated by less than 6 A but are nearly insignificant above 7 A. Aromatic-aromatic side chain interactions are on the order of -1.0 to -1.4 kcal/mol and do not appear to depend on the type of aromatic residue. These results suggest that the clustering of aromatic side chains at membrane interfaces provides an additional heretofore not yet recognized driving force for the folding and stability of integral membrane proteins.     

Comparison between monolithic conventional size, microbore and capillary poly(p-methylstyrene-co-1,2-bis(p-vinylphenyl)ethane) high-performance liquid chromatography columns Synthesis, application, long-term stability and reproducibility

Novel monolithic supports (MS/BVPE) were prepared by thermally initiated free radical copolymerisation of p-methylstyrene (MS) and 1,2-bis(p-vinylphenyl)ethane (BVPE). The polymer was synthesised in fused silica capillaries (80 mm x 0.2 mm and 80 mm x 0.53 mm) and in borosilicate glass columns (90 mm x 1.0 mm and 90 mm x 3.0 mm) to yield different HPLC column designs. A comparison of those column dimensions regarding morphology as well as separation efficiency and applicability in bioanalysis is presented. The efficiency towards proteins as well as oligonucleotides was found to be considerably improved with decreasing column I.D. While a 5-protein mixture was baseline separated on all investigated column designs, the separation of small biomolecules like oligonucleotides or peptides on microbore and conventional size glass columns was strongly restricted in terms of resolution due to extensive peak broadening or the occurrence of peak asymmetry. Monolithic MS/BVPE capillary columns up to 0.53 mm I.D., however, proved to be applicable to the fractionation of the whole spectrum of biopolymers, including proteins, peptides, oligonucleotides as well as double-stranded DNA fragments. Due to the fact that reliable chromatography makes great demand on the robustness of the stationary phase, monolithic MS/BVPE capillaries were subjected to a comprehensive reproducibility study including run-to-run as well as batch-to-batch reproducibility.