Mass spectra of alditol trifluoroacetates

The trifluoroacetyl derivatives of alditols are convenient compounds for mass spectrometric investigation as they are easily obtainable and highly volatile. They show simple fragmentation patterns with intensive peaks in the high mass range. Trifluoroacetates of alditols may be used for detection and location of deoxy groups in the molecule.     

Determination of affinity constants and response factors of the noncovalent dimer of gramicidin by electrospray ionization mass spectrometry and mathematical modeling

The dimerization of gramicidin, a 15-residue membrane peptide, in solution can be viewed as a model for protein-protein interactions. We reported previously that the dimer can be observed when electrosprayed from organic solvents and that the abundances of the dimer depends on the dielectric constant of the solvent. Here, we report an effort to determine an affinity constant for the dimerization of gramicidin by using gas-phase abundance. Two issues affecting the determination are the electrospray-induced dissociation of the dimer and discrimination in the electrospray of the dimer compared with the monomer. Other methods developed for the purpose of determining affinity from mass spectral abundance do not address the dissociation of the complex in the gas phase or can not be applied for cases of low affinity constant, K(a). We present a mathematical model that uses the ratio of the signal intensities of the dimer and the monomer during a titration. The model also incorporates the dissociation and an electrospray ionization-response factor of the dimer for extracting the affinity constant for the dimerization of gramicidin. The dimerization constants from the new method agree within a factor of two with values reported in the literature.     

Investigation in the Series of Substituted Butan- and Butenolides: XV. Transformations of 4-Hydroxy-2-butenolide in Water Media at Various pH Values

The 4-hydroxy-2-butenolide was established by polarographic method to undergo in water solutions fast tautomeric and acid-base transformations. Depending on pH of the medium the compound is present either as a cyclic irreducible form (at pH 0-4) or in the open-chain (carbonyl-containing) reducible forms (at pH > 4 and < 0). It is presumed that in going from the basic to strongly acidic medium and backwards occur consecutive equilibrium transitions of four species: cis--formylacrylic acid anion (A^-), its neutral molecule (HA) existing as linear and cyclic tautomers, and protonated forms (H₂A⁺ and H₃A⁺). The formation mechanism thereof is considered.     

Determination of organic alkyl- and 1-hydroxy hydroperoxides with chemiluminescence, fluorescence and GC/MS

Summary In this study three methods for the determination of peroxy compounds with HPLC are presented. The applicability of these methods with respect to atmospheric chemistry is discussed. In rain samples 1-hydroxy hydroperoxides are determined with a detection limit of 0.058 mol/l. Alkyl hydroperoxides are determined in a mixture of hydrocarbon, hydrogen peroxide and air which has been irradiated with light before analysis. A new method for the detection of peroxides in such photolysis mixtures by GC/MS is presented.     

Photochemical thallimetric oxidations-estimation of formic acid

A simple, rapid and convenient redox method has been developed for the estimation of formic acid. Formic acid is photochemically oxidized with thallium(III) in the presence of bromide as catalyst, and the thallium(I) formed is determined by titration with potassium bromate. The procedure can also be used for the estimation of thallium(III) with formic acid as reductant.     

New pyrazino[1,2-a]benzimidazole derivatives

1-Ethoxycarbonyl-3-phenyl- and 3-phenylpyrazino[1,2-a]benzimidazoles were synthesized by reaction of 2-ethoxalyl- and 2-diethoxymethyl-N-phenacylbenzimidazoles, obtained from the corresponding sodium derivatives of benzimidazole and phenacyl bromide, with ammonium acetate.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 9, pp. 1266–1267, September, 1976.     

Cumulated double bond systems as ligands : V. Dialkylsulfurdiimine compounds M(CO)4(RNSNR) (M = Cr,Mo, W) and M(CO)5(RNSNR) (M = Cr,W); structural,spectroscopic and fluxional properties.

The preparation and properties are reported of M(CO)₄(RNSNR) (M = Cr, Mo, W; R = i-Pr, t-Bu), in which the ligand is bidentate and in the trans,trans configuration, and of M(CO)₅(RNSNR) (M = CR, W; R = Et, i-Pr) in which the sulfurdiimine is monodentate and in the cis,trans configuration. In both cases the ligand is linked to the metal atom via the N-atom(s). With M(CO)₅(MeNSNMe) a second isomer is found in which the sulfurdiimine is probably bonded via the S-atom to the metal. All the pentacarbonyl compounds are fluxional; this is attributed to a gliding movement of the metal atom along the NSN system.Both W(CO)₄(t-BuNSN-tBu) and W(CO)₅(MeNSNMe) show vibronic coupling of metal to ligand charge transfer transitions with sulfurdiimine vibrations, as shown with Resonance Raman, but only for W(CO)₅(MeNSNMe) also with the symmetric mode of the equatorial carbonyl groups. The metalsulfurdiimine bond appears to be weak for M(CO)₅(RNSNR), but strong for M(CO)₄(RNSNR).     

The Effect of Solute Concentration on Equilibrium Partitioning in Polymeric Gels

The effect of solute concentration on the equilibrium partitioning of sphere-like, colloidal solutes in stiff polymer hydrogels is examined theoretically and experimentally. The theoretical development is a statistical mechanics approach, and allows quantitative calculations to be performed to determine the concentration-dependent partition coefficient correct to first order in solute concentration at specific surface charge densities. The theory predicts that repulsive steric and/or electrostatic solute-fiber interactions exclude solute from the gel phase, but that repulsive solute-solute interactions cause partitioning into the gel to increase with increasing solute concentration. These trends are enhanced for larger solutes, increased fiber volume fractions, or stronger electrostatic repulsion. Partition coefficients have also been measured for two proteins, bovine serum albumin (BSA) and alpha-lactalbumin (ALA), in a system consisting of a salt solution and cubes of agarose hydrogel. To investigate the effect of electrostatic interactions, the experiments were performed at 0.15 M KCl and 0.01 M KCl. The theory underpredicts the strong electrostatic repulsion between BSA macromolecules at the lower ionic strength. The experimental results for ALA show the influence of an attractive interaction between the protein macromolecules, in addition to hard-sphere repulsive and electrostatic interactions. Copyright 2001 Academic Press.     

A symmetry-based formal synthesis of zaragozic acid A

A symmetry-based strategy for the synthesis of the zaragozic acids is reported. Two enantioselective dihydroxylations were used to establish the absolute configuration of a C(2) symmetric intermediate. Noteworthy transformations include a group-selective lactonization, which accomplished an end-differentiation of a pseudo-C(2) symmetric intermediate. Late stage protecting group adjustments and oxidations accomplished a formal synthesis of zaragozic acid A.