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.     

Catalytic Asymmetric Aminohydroxylation with Amino-Substituted Heterocycles as Nitrogen Sources

The suprafacial, vicinal addition of a heterocyclic moiety and a hydroxyl group is achieved by the osmium-catalyzed asymmetric aminohydroxylation (AA) of olefins with amino-substituted heterocycles as the nitrogen sources. Amino alcohols are obtained in up to 97 % yield and with up to 99 % ee when a ligand derived from dihydroquinidine (DHQD-L) is used [Eq. (1); Rⁱ indicates the remaining portion of the heterocycle (Het); H₂O is the O source]. The AA can now be considered as a means to directly introduce complex, biologically relevant substructures to hydrocarbon backbones.     

Carbon−Fluorine Bond Formation via a Five-Coordinate Fluoro Complex of Ruthenium(II), Preliminary Communication

The 16-electron, five-coordinate fluoro complex [RuF(dppp)₂]PF₆ ( 1a ; dppp=propane-1,3-diylbis[diphenylphosphine] smoothly reacts with 1,3-diphenylallyl bromide (=1,1′-(3-bromoprop-1-ene-1,3-diyl)bis[benzene]) in dry CDCl₃ to give 1,3-diphenylallyl fluoride and [RuBr(dppp)₂]⁺ in nearly quantitative yield. Under similar conditions, bromide (or chloride)/fluoride exchange also occurs with chlorotriphenylmethane, bromodiphenylmethane, and tert-butyl bromide. The crystal structure of 1a is reported.     

Laser ablation of energetic polymer solutions: effect of viscosity and fluence on the splashing behavior

Laser plasma thrusters are a new kind of propulsion system for small satellites, and work with the thrust created by the laser ablation of a target. Liquid polymer solutions are very promising fuels for such systems, provided that no splashing of the target occurs, because ejection of droplets strongly decreases the performances of the system. We have investigated the nanosecond infrared laser ablation of glycidyl azide polymer solutions containing carbon nanoparticles as absorber. Shadowgraphy imaging revealed two cases, namely splashing regime and solid-like behavior. The transition between both regimes depends on the viscosity of the solution and on the laser fluence, and is explained by the recoil force acting on the target. Appropriate conditions to avoid splashing were identified, showing that this liquid polymer solution is a suitable fuel for laser plasma thrusters.