Synthesis,characterization, X-ray structural analysis and study of oxidative properties of propyltriphenylphosphonium bromochromate

The complex of propyltriphenylphosphonium bromochromate(VI), PrPh₃P[CrO₃Br] is easily synthesized in nearly quantitative yield using a direct reaction of chromium(VI) oxide and propyltriphenylphosphonium bromide. This compound is a versatile reagent for the efficient and selective oxidation of organic substrates, in particular for alcohols to their corresponding aldehydes or ketones, under mild conditions. This compound was characterized by IR, UV/Visible, ¹³C-n.m.r. and ¹H-n.m.r. techniques. It crystallized in the monoclinic form and its crystal and molecular structure have been determined by X-ray crystallography.     

Multigrid solution of the Poisson—Boltzmann equation

A multigrid method is presented for the numerical solution of the linearized Poisson–Boltzmann equation arising in molecular biophysics. The equation is discretized with the finite volume method, and the numerical solution of the discrete equations is accomplished with multiple grid techniques originally developed for twodimensional interface problems occurring in reactor physics. A detailed analysis of the resulting method is presented for several computer architectures, including comparisons to diagonally scaled CG, ICCG, vectorized ICCG and MICCG, and to SOR provided with an optimal relaxation parameter. Our results indicate that the multigrid method is superior to the preconditioned CG methods and SOR and that the advantage of multigrid grows with the problem size. © 1993 John Wiley & Sons, Inc.     

Hydroxyl radical probe of the calmodulin-melittin complex interface by electrospray ionization mass spectrometry

The calcium-dependent interaction of calmodulin and melittin is studied through the application of a radical probe approach in which solutions of the protein and peptide and protein alone are subjected to high fluxes of hydroxyl and other oxygen radicals on millisecond timescales. These radicals are generated by an electrical discharge within an electrospray ion source of a mass spectrometer. Condensation of the electrosprayed droplets followed by proteolytic digestion of both calmodulin and melittin has identified residues in both which participate in the interaction and/or are shielded from solvent within the protein complex. Consistent with other theoretical models and available experimental data, the tryptophan residue of melittin at position 19 is shown to be critical to the formation of the complex with the C-terminal domain of peptide enveloped by and protected from oxidation upon binding to the protein. Furthermore, the N-terminal domain (to residue 36) and tyrosine at position 99 in calmodulin are significantly protected from limited oxidation upon the binding of melittin while exposing the phenylalanine residue at position 92 of the flexible loop domain. The N-terminus (through residue 36) of calmodulin is shown to lie in closer proximity to the melittin helix than its C-terminal counterpart (residues 127-148) based upon the protection levels measured at reactive residues within these segments of the protein.     

Electrospray-assisted modification of proteins: a radical probe of protein structure

A new approach is described to probe the structure of proteins through their reactivity with oxygen-containing radicals. Radical-induced oxidative modification of proteins is achieved within an electrospray ion source using oxygen as a reactive nebulizer gas at high needle voltages. This method facilitates the rapid oxidation of proteins as the molecules emerge from the electrospray needle tip. Electrospray mass spectra of both ubiquitin and lysozyme reveal that over 50% of the protein can be modified under these conditions. The radical-induced oxidative modification of amino acid side chains is correlated with their solvent accessibility to obtain information on a protein's higher-order structure. The oxidation sites in hen lysozyme have been identified by proteolysis of the condensed protein solution and tandem mass spectrometry (MS/MS). Oxidation of tryptophan at positions 62 and 123 occurs exclusively over all other tryptophan residues, consistent with the relative solvent accessibilities of the residue side chains based on the NMR structure of the protein. Radical-induced oxidative modification of cysteine (Cys), methionine (Met), tryptophan (Trp), phenylalanine (Phe), tyrosine (Tyr), proline (Pro), histidine (His), and leucine (Leu) residues is also reported, providing sufficient reactive markers to span a protein sequence. This facile oxidation process could be applied to investigate the molecular mechanism by which reactive oxygen species interact with a particular protein domain as a means to investigate the onset of certain diseases.     

Solvent selection in ultrasonic-assisted emulsification microextraction: Comparison between high- and low-density solvents by means of novel type of extraction vessel

There are numerous published reports about dispersive liquid phase microextraction of the wide range of substances, however, till now no broadly accepted systematic and purpose oriented selection of extraction solvent has been proposed. Most works deal with the optimization of available solvents without adequate pre-consideration of properness. In this study, it is tried to compare the performances of low- and high-density solvents at the same conditions by means of novel type of extraction vessel with head and bottom conical shape. Extraction efficiencies of seven basic pharmaceutical compounds using eighteen common organic solvents were studied in this work. It was much easier to work with high-density solvents and they mostly showed better performances. This work shows that although exact predicting the performance of the solvents is multifaceted case but the pre-consideration of initial selection of solvents with attention to the physiochemical properties of the desired analytes is feasible and promising. Finally, the practicality of the method for extraction from urine and plasma samples was investigated.     

Surface modified SPIONs‐Cr(VI) ions‐immobilized organic‐inorganic hybrid as a magnetically recyclable nanocatalyst for rapid synthesis of polyhydroquinolines under solvent‐free conditions at room temperature

In this work, a magnetic hybrid dichromate nanocomposite with triphenylphosphine surface modified superparamagnetic iron oxide nanoparticles (SPIONs) as a recyclable nanocatalyst was designed, prepared and characterized by Fourier transform infrared spectroscopy (FT‐IR) spectra, X‐ray diffraction (XRD) pattern analysis, vibrating sample magnetometer (VSM) curves, X‐ray fluorescence (XRF) analysis, thermogravimetric analysis (TGA), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images and dynamic light scattering (DLS) analysis. Then, it was used in a green and efficient procedure for one‐pot multicomponent synthesis of polyhydroquinoline derivatives by the condensation of aldehydes, dimedone or 1,3‐cyclohexadione, ethyl acetoacetate and ammonium acetate. This protocol includes some new and exceptional advantages such as short reaction times, low catalyst loading, high yields, solvent‐free and room temperature conditions, easy separation and reusability of the catalyst.     

Convenient synthesis of substituted benzo[e][1,2,4]- or [d][1,2,6]oxadiazepines,benzo[f][1,3,5]triazocines from N-aryliminoesters

We report herein a short and efficient synthesis of benz[e][1,2,4]- or [d][1,2,6]oxadiazepines and benzo[f][1,3,5]triazocines from easily prepared N-aryl iminoesters. The strategy involves a bis-nucleophile reagent (hydroxylamine or guanidine) that promotes a one-step ring closure from the starting functionalized iminoesters.