Modelling migration behavior of peptide hormones in capillary electrophoresis-electrospray mass spectrometry

The applicability in capillary electrophoresis-electrospray mass spectrometry (CE-ESI-MS) of the classical semiempirical relationships between electrophoretic mobility and charge-to-mass ratio (me versus q/Malpha) has been investigated in order to describe the migration behavior of a series of bioactive peptide hormones. The influence upon the models of the separation electrolyte pH and the accuracy of the pK values of these compounds were studied first by capillary electrophoresis with ultraviolet detection (CE-UV). The classical polymer model, alpha = 1/2, resulted in slightly better correlations at any of the studied pH. Furthermore, a general linear equation can be adjusted combining all the experimental data pairs, which suggests that correlation in the whole pH range is independent of the ionic form of the studied peptide hormones. The plots of q/M1/2 against separation electrolyte pH were used to predict their electrophoretic separations, using the accurate pK values obtained in a previous work by CE-UV for charge calculations. A volatile separation electrolyte containing 50 mM of acetic acid and 50 mM of formic acid at pH 2.85 was selected for optimum CE-UV and CE-ESI-MS analysis of the peptide mixture. At this pH and taking into account the specific features of the coupling, the correlation using the classical polymer law was excellent and its parameters were similar to the ones of the general linear equation previously obtained by CE-UV. This confirmed the applicability in CE-ESI-MS of the semiempirical relationship originally established by CE-UV.     

Apolar chromatography on Sephadex LH-20 combined with high-speed counter-current chromatography. High yield strategy for structurally closely related analytes-Destruxin derivatives from Metarhizium anisopliae as a case study

A novel high yield isolation procedure for lipophilic cyclic peptide derivatives is presented. Destruxin (dtx) A, B, D, E, and E-diol retrieval from Metarhizium anisopliae culture broth was achieved with a three-step purification protocol. After liquid-liquid extraction column chromatography over Sephadex LH-20 served as enrichment step. High-speed counter-current chromatography (HSCCC) was used for the final purification. Within the first chromatographic step dtx D and dtx E-diol were separated in purities exceeding 90%. The separation of dtx A, B, and E was achieved from an enriched Sephadex LH-20 fraction by a HSCCC protocol using light petroleum-ethyl acetate-methanol-water = 2:5:2:5 (v/v) as eluent system. These derivatives were obtained in purities above 98% and total yields exceeding 40%.     

Quantification of Fumaria officinalis isoquinoline alkaloids by nonaqueous capillary electrophoresis-electrospray ion trap mass spectrometry

A capillary electrophoresis (CE) method using non-aqueous (NA) separation solutions combined with an ion trap mass spectrometer (MS and MS/MS) as detection device is presented for the separation, identification and quantification of isoquinoline alkaloids from Fumaria officinalis. The best results were obtained with a mixture of acetonitrile-methanol (9:1, v/v) containing 60mM ammonium acetate and 2.2M acetic acid as running electrolyte and an applied voltage of 30 kV. Electrospray MS measurements were performed in the positive ionization mode with isopropanol-water (1:1, v/v) as sheath liquid at a flow rate of 3 microl/min. Alkaloids were detected as [M+H](+)-ions and showed typical fragmentation patterns in MS/MS experiments. The developed assay was used for the quantification of seven isoquinoline alkaloids representing different structural subtypes in Fumariae herba extracts and F. herba containing phytopharmaceuticals.     

Layered hybrid organic-inorganic Co(II) alkylphosphonates. Synthesis, crystal structure and magnetism of the first two members of the series: Co[(CH3PO3)(H2O)] and Co[(C2H5PO3)(H2O)]

Co[(CH₃PO₃)(H₂O)] (1) and Co[(C₂H₅PO₃)(H₂O)] (2) were prepared by the hydrothermal method and isolated as blue-violet platelet crystals. They were characterized by X-ray diffraction, FT-IR, TGA-DSC techniques and their magnetic properties studied by a dc-SQUID magnetometer. Compound (1) shows an hybrid layered structure, made of alternating inorganic and organic layers along the a-direction of the unit cell. The inorganic layers contain Co(II) ions six-coordinated by five phosphonate oxygen atoms and one from the water molecule. These layers are separated by bi-layers of methyl groups and van der Waals contacts are established between them. In compound (2), the layered hybrid structure is rather similar to that described for compound (1), but the alternation of the inorganic and organic layers is along the b-direction of the unit cell. The magnetic behavior of (1) and (2) as function of temperature and magnetic field was studied. The compounds obey the Curie-Weiss law at temperatures above 100 K, the Curie C, and Weiss θ constants for the methyl derivative being and and for the ethyl derivative and , respectively. The observed magnetic moments for Co atom at room temperature (i.e. μ_eff=5.18 and 5.38 BM, respectively) are higher than those expected for a spin-only value for high spin Co(II) (S=3/2), revealing a substantial orbital contribution to the magnetic moment. The negative values of θ are an indication of the presence of antiferromagnetic exchange couplings between the near-neighbors Co(II) ions, within the layers. [Co(C_nH_2n+1PO₃)(H₂O)] (n=1,2) are 2D Ising antiferromagnets at low temperatures.     

Solid-state laser spectral narrowing using a volumetric photothermal refractive Bragg grating cavity mirror

Dramatic spectral narrowing of two normally broadband lasers, Ti:sapphire and Cr:LiSAF, was achieved by simply replacing the output mirror with a reflective, volumetric Bragg grating recorded in photothermal refractive glass. The output power of each laser was unchanged from that obtained using dielectric coated output mirrors with the same output coupling as the Bragg grating while spectral brightness increased by 3 orders of magnitude.     

Insulator, semiclassical oscillations and quantum Hall liquids at low magnetic fields

Magneto-transport measurements are performed on two-dimensional GaAs electron systems to probe the quantum Hall (QH) effect at low magnetic fields. Oscillations following the Shubnikov-de Haas (SdH) formula are observed in the transition from the insulator to QH liquid when the observed almost temperature-independent Hall slope indicates insignificant interaction correction. Our study shows that the existence of SdH oscillations in such a transition can be understood based on the non-interacting model.     

Ultrasonic velocity and amplitude characterization of magnetorheological fluids under magnetic fields

Variations in velocity of sound and amplitude of the signal of a commercial magnetorheological fluid under different magnetic fields are studied experimentally. Different factors such as orientation, uniformity, geometry and intensity of the magnetic field are investigated. An increase in the change of MR fluid acoustical properties is obtained when the magnetic field intensity is risen. In addition, these properties show an opposite behavior when a magnetic field is applied parallel or perpendicular to the ultrasound propagation. Experiments using an electromagnet and permanent magnets as the source of magnetic field are also compared. Properties such as anisotropy in sound velocity and amplitude make these materials interesting regarding applications.     

Fitting properties from density functional theory based molecular dynamics simulations to parameterize a rigid water force field

In the quest towards coarse-grained potentials and new water models, we present an extension of the force matching technique to parameterize an all-atom force field for rigid water. The methodology presented here allows to improve the matching procedure by first optimizing the weighting exponents present in the objective function. A new gauge for unambiguously evaluating the quality of the fit has been introduced; it is based on the root mean square difference of the distributions of target properties between reference data and fitted potentials. Four rigid water models have been parameterized; the matching procedure has been used to assess the role of the ghost atom in TIP4P-like models and of electrostatic damping. In the former case, burying the negative charge inside the molecule allows to fit better the torques. In the latter, since short-range interactions are damped, a better fit of the forces is obtained. Overall, the best performing model is the one with a ghost atom and with electrostatic damping. The approach shown in this paper is of general validity and could be applied to any matching algorithm and to any level of coarse graining, also for non-rigid molecules.     

Granular flow from a silo: Discrete-particle simulations in three dimensions

Molecular (or granular) dynamics methods are used to study the gravity-driven flow of granular material through a horizontal aperture in three dimensions. The grains are spherical and modeled using a short-range repulsive interaction, together with normal and tangential frictional damping forces. The material is contained in a rough-walled cylindrical container with a circular hole in its base, and to permit flow measurements under steady-state conditions a continuous feed approach is employed in which exiting grains are replaced at the upper surface of the material. The dependence of flow velocity and discharge rate on aperture diameter is found to agree with experiment; other quantities such as the kinetic energy and pressure distributions are also examined. Received 5 June 2000 and Received in final form 21 September 2000