Charge-state reduction with improved signal intensity of oligonucleotides in electrospray ionization mass spectrometry

The shift of charge states of oligonucleotide negative ions formed in electrospray ionization mass spectrometry to higher mass-to-charge ratio has been accomplished by addition of organic acids and bases to the solution to be electrosprayed. The use of acetic acid or formic acid combined with piperidine and imidazole effectively reduced charge states. Signal intensity and stability were enhanced greatly when the infused solution contained a high percentage of acetonitrile. In addition, the cocktail that contained imidazole, piperidine, and acetic acid in 80% acetonitrile not only reduced charge states, but also substantially suppressed Na adduction. Several oligonucleotides that varied in base composition and length were investigated, and studies of mixtures showed a significant reduction in spectral complexity.     

A minimal area problem in conformal mapping

LetS denote the usual class of functionsf holomorphic and univalent in the unit diskU such thatf(0)=f′(0)−1=0. The main result of the paper is that area (f(U) ≥27π/7)(2-α)⁻² for allf∈S such that |f″(0)|=2α, 1/2<α<2. This solves a long-standing extremal problem for the class of functions considered.     

Molecular simulation on hydrodesulfurization of thiophenic compounds over MoS2 using ZINDO

In order to develop a fundamental understanding of the HDS mechanism of thiophenic compounds over molybdenum disulfide (MoS₂), a molecular simulation of the hydrodesulfurization (HDS) of thiophenic compounds over MoS₂ has been performed using Zerner's Intermediate Neglect of Differential Overlap (ZINDO) program. On the basis of the calculated edge structure, stoichiometry of MoS₂, shape of the crystal, and the size corresponding to real MoS₂ particles, a single-slab cluster, Mo₂₇S₅₄, has been proposed for modeling the highly dispersed MoS₂. The proposed cluster is a regular hexagon with ( 010) and (30 0) edge planes only. According to the calculated electronic properties of the surface, the coordinately unsaturated Mo_IV in the (30 0) plane is expected to be the active site for hydrogenation of thiophenic and aromatic compounds. The most stable adsorption configuration of thiophene on the Mo_IV is a flat adsorption configuration via the η⁵-bound coordination, whereas the most stable adsorption configuration of tetrahydrothiophene (THT) on the Mo_IV is a tilted adsorption configuration via the S-bound coordination. HDS mechanism of thiophene through the hydrogenation pathway over the (30 0) plane of MoS₂ is discussed according to quantum chemical insights in combination with experimental results from the literature.     

An intramolecularly stabilized arylboron dibromide

A new boron dibromide featuring intramolecular Lewis base coordination, ArBBr₂ ( 4; Ar = 2-[(dimethylamino)methyl]phenyl) has been prepared via the salt-elimination reaction of [2-(Me₂NCH₂)C₆H₄]Li with BBr₃. Single-crystal X-ray diffraction experiments reveal that 4 rystallizes in two polymorphs, both of which are orthorhombic. The principal difference between the two polymorphs is the presence or absence of weak Br…Br intermolecular contacts. The metrical parameters for the individual molecules of 4 are very similar for both polymorphs; the amine “arm” is coordinated strongly, and the boron atom adopts a somewhat distorted tetrahedral geometry. Compound 4 is surprisingly unreactive toward a variety of hydride transfer reagents. © 1998 John Wiley & Sons, Inc. Heteroatom Chem 9:79–83, 1998     

A fast adaptive multigrid boundary element method for macromolecular electrostatic computations in a solvent

A fast multigrid boundary element (MBE) method for solving the Poisson equation for macromolecular electrostatic calculations in a solvent is developed. To convert the integral equation of the BE method into a numerical linear equation of low dimensions, the MBE method uses an adaptive tesselation of the molecular surface by BEs with nonregular size. The size of the BEs increases in three successive levels as the uniformity of the electrostatic field on the molecular surface increases. The MBE method provides a high degree of consistency, good accuracy, and stability when the sizes of the BEs are varied. The computational complexity of the unrestricted MBE method scales as O(N_at), where N_at is the number of atoms in the macromolecule. The MBE method is ideally suited for parallel computations and for an integrated algorithm for calculations of solvation free energy and free energy of ionization, which are coupled with the conformation of a solute molecule. The current version of the 3-level MBE method is used to calculate the free energy of transfer from a vacuum to an aqueous solution and the free energy of the equilibrium state of ionization of a 17-residue peptide in a given conformation at a given pH in ∼ 400 s of CPU time on one node of the IBM SP2 supercomputer. © 1997 by John Wiley & Sons, Inc. J Comput Chem 18: 569–583, 1997     

New synthetic routes to poly(isothianaphthene). II. Mechanistic aspects of the reactions of phthalic anhydride and phthalide with phosphorus pentasulfide

The results of some mechanistic studies on the formation of poly(isothianaphthene) from phthalic anhydride and phthalide by reaction with phosphorus pentasulfide (P₄S₁₀) are described. Based on the observed intermediates during the polymerization and their reactivity, it is proposed that both reactions occur by a sequence of substitution (thionation), isomerization, and polymerization reactions. P₄S₁₀ is the most efficient reagent for the conversion of phthalic anhydride and phthalide, and Lawesson's Reagent (a commonly used thionating reagent) gives less satisfactory results. It is suggested that P₄S₁₀ assists the rate-determining step. Oxygen-containing monomers do not polymerize in the absence of a thionating reagent under the conditions for the synthesis of PITN, thereby keeping the incorporation of oxygen into the polymeric backbone to a minimum. © 1996 John Wiley & Sons, Inc.     

Emulsion co- and terpolymerization of styrene,methyl methacrylate and methyl acrylate. II. Control of emulsion terpolymer microstructure with optimal addition profiles

An optimal addition profile for the preparation of a chemically homogeneous emulsion terpolymer of styrene, methyl methacrylate, and methyl acrylate was determined using a recently developed model for describing composition drift in emulsion co- and terpolymerizations. TRISEPS, described in Part I of this series. The model uses recently published simplified equations to describe monomer partitioning and the terminal model for describing terpolymer composition. The optimal addition rate profile was determined from the calculated optimal addition profile with a purely empirical and iterative method. With gradient polymer elution chromatography (GPEC®) the homogeneity and/or heterogeneity of the terpolymers prepared in the iterative series of experiments could be determined and compared to the heterogeneity of the corresponding batch terpolymer described in Part I. It was shown that a homogeneous terpolymer could be obtained indicating that the simplified equations for monomer partitioning and the terminal model for terpolymer composition describe the system adequately. It was also shown that GPEC® was useful in the determination of the optimal addition rate profile. © 1996 John Wiley & Sons, Inc.