Operational variables in high-performance liquid chromatography-electrospray ionization mass spectrometry of peptides and proteins using poly(styrene-divinylbenzene) monoliths

Capillary reversed-phase high-performance liquid chromatography (RP-HPLC) utilizing monolithic poly(styrene-divinylbenzene) columns was optimized for the coupling to electrospray ionization mass spectrometry (ESI-MS) by the application of various temperatures and mobile phase additives during peptide and protein analysis. Peak widths at half height improved significantly upon increasing the temperature and ranged from 2.0 to 5.4 s for peptide and protein separations at 70 degrees. Selectivity of peptide elution was significantly modulated by temperature, whereas the effect on proteins was only minor. A comparison of 0.10% formic acid (FA), 0.050% trifluoroacetic acid (TFA), and 0.050% heptafluorobutyric acid (HFBA) as mobile phase additives revealed that highest chromatographic efficiency but poorest mass spectrometric detectabilities were achieved with HFBA. Clusters of HFBA, water, and acetonitrile were observed in the mass spectra at m/z values >500. Although the signal-to-noise ratios for the individual peptides diverged considerably both in the selected ion chromatograms and extracted mass spectra, the average mass spectrometric detectabilities varied only by a factor of less than 1.7 measured with the different additives. Limits of detection for peptides with 500 nl sample volumes injected onto a 60 mm x 0.20 mm monolithic column were in the 0.2-13 fmol range. In the analysis of hydrophobic membrane proteins, HFBA enabled highest separation selectivity at the cost of lower mass spectral quality. The use of 0.050% TFA as mobile phase additive turned out to be the best compromise between chromatographic and mass spectrometric performance in the analysis of peptides and proteins by RP-HPLC-ESI-MS using monolithic separation columns.     

Vibrational and high pressure conformational studies of 1,1,2-trichloroethane

The infrared spectra of 1,1,2-trichloroethane were recorded from 4000 to 50 cm^?1 in the vapour and liquid states. Additional spectra above 200 cm^?1 of the low temperature crystal and of two crystalline solids, prepared by compressing the sample in a diamond anvil cell, were obtained. Also, infrared spectra of the liquid and of the compound dissolved in CS₂ were recorded at increased pressures. Raman spectral data of the liquid (including polarization measurements) and of the low temperature crystal were obtained.The fundamental frequencies for each of the two conformers, C₁ and C_s, were assigned and the results checked by normal coordinate analysis. The same diagonal and off-diagonal force constants were employed for the two conformers and the force fields were derived by means of a least squares refinement, including data for chloroethane, 1,1-dichloroethane and various deuterated species. A standard deviation of 3 % was obtained in the final fit.From the changes in relative intensities for infrared bands belonging to the C₁ and C_s conformers with increasing pressure, the volume differences (ΔV¯between the conformers were determined in CS₂ solution and in the pure liquid.     

Strukturen ladungsgestörter oder räumlich überfüllter Moleküle. 15. Tetracyanethen-Natrium-Dimethoxyethan

Structures of Charge-Perturbed or Sterically Overcrowded Molecules. 16. Tetracyanoethylene Sodium Dimethoxyethane The Single crystal structure of [(NC)₂C? C(CN)₂?·Na⊕(H₃CO? CH₂CH₂? OCH₃)]∞ reveals two formula units within the triclinic (P1 ) unit cell. The tetracyanoethylene radical anions are arranged along parallel double layers, which are shifted relative to each other, and in between which are interspersed the sodium counter cations and their dimethoxyethane ligands. The distances within the double layers amount to 300 pm and the ones between them to 385 pm. The six-fold coordinated Na⊕ centers are surrounded by four radical anions with contact distances Na…?N between 250 and 254 pm as well as by a twofold solvent ligand with Na…?O of 238 and 241 pm. Due to the electron transfer to the acceptor molecule, its (NC)₂C-halves twist by 8° and the bond lengths of the N?C? C subunits, bent by each 3°, are shortened up to 2 pm. The structural parameters are compared to those of the analogous potassium salt [TCNE^?K^⊕DME], of the dianion , of the sodium salts [(NC)₃C^?Na^⊕]∞ as well as [(NC)₂C? C(CHCH)₂? C(CN)₂^?Na^⊕]_∞ and, in addition, are discussed based on geometry-optimized MNDO calculations.     

Substituent control of hydrogen bonding in palladium(II)-pyrazole complexes

Inter- and intramolecular hydrogen bonding of an N-H group in pyrazole complexes was studied using ligands with two different groups at pyrazole C-3 and C-5. At C-5, groups such as methyl, i-propyl, phenyl, or tert-butyl were present. At C-3, side chains L-CH(2)- and L-CH(2)CH(2)- (L = thioether or phosphine) ensured formation of chelates to a cis-dichloropalladium(II) fragment through side-chain atom L and the pyrazole nitrogen closest to the side chain. The significance of the ligands is that by placing a ligating side chain on a ring carbon (C-3), rather than on a ring nitrogen, the ring nitrogen not bound to the metal and its attached proton are available for hydrogen bonding. As desired, seven chelate complexes examined by X-ray diffraction all showed intramolecular hydrogen bonding between the pyrazole N-H and a chloride ligand in the cis position. In addition, however, intermolecular hydrogen bonding could be controlled by the substituent at C-5: complexes with either a methyl at C-5 or no substituent there showed significant intermolecular hydrogen bonding interactions, which were completely avoided by placing a tert-butyl group at C-5. The acidity of two complexes in acetonitrile solutions was estimated to be closer to that of pyridinium ion than those of imidazolium or triethylammonium ions.     

Rare Events in Random Geometric Graphs

This work introduces and compares approaches for estimating rare-event probabilities related to the number of edges in the random geometric graph on a Poisson point process. In the one-dimensional setting, we derive closed-form expressions for a variety of conditional probabilities related to the number of edges in the random geometric graph and develop conditional Monte Carlo algorithms for estimating rare-event probabilities on this basis. We prove rigorously a reduction in variance when compared to the crude Monte Carlo estimators and illustrate the magnitude of the improvements in a simulation study. In higher dimensions, we use conditional Monte Carlo to remove the fluctuations in the estimator coming from the randomness in the Poisson number of nodes. Finally, building on conceptual insights from large-deviations theory, we illustrate that importance sampling using a Gibbsian point process can further substantially reduce the estimation variance.

     

An apparatus for the determination of volatile analytes by stopped-flow injection analysis using an integrated fiber optic detector

A new method for the analysis of volatile analytes using a stopped-flow injection system originating from either a gas or liquid phase has been developed. It uses an integrated fiber optic detector which also serves as a reactor. This system combines the advantages of gas diffusion and stopped-flow, making the overall assay very sensitive. Both gas streams and aqueous solutions containing ammonia were analyzed. The limits of detection are 40 ppb for gas phase analysis and 1.0 ppm for aqueous phase analysis.     

Ergodicity of a Class of Cocycles Over Irrational Rotations

It is proved that if is irrational and L²(S¹) with o(l/n)then for each mZ\{0} the corresponding skew product is ergodic. The rigidity of specialflows over irrational rotations with roof functions whose Fouriercoefficients are in o(l/n) is also shown.     

Sur le comportement asymptotique des potentiels d'une chaîne de Markov sur ℝ d

Résumé Dans cet article j'étudie le comportement à l'infini des potentiels des chaînes de Markov sur ^d (d3) proches du mouvement brownien, tout spécialement le cas des marches aléatoires, ainsi que des critères de transience et de récurrence inspirés de la méthode utilisée.

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We study the asymptotic behaviour of potentials of Markov chains on ^d (d3), closed to Brownian motion, and particularly the case of random walks. Following a similar approach, we give transience and recurrence criteria.