Vacancies in the solids of low molecular weight organic compounds observed by positron annihilation

PAL were measured for several low molecular organic compounds, normal and cyclic-hydrocarbons and their perfluorinated ones, from room temperature down to about 40 K, and information about the vacancies in them has been extracted from ₃ andI ₃. Normally the size of vacancies were larger in the solids comprising larger molecules, showing thato-Ps can represent the vacancy size. In a special case of perfluorocyclohexane whose solid had to be prepared by sublimation the vacancy size was larger than expected. In all the solid molecules studied twoo-Ps states could be observed and their relative importance could easily change showing a hysteresis-like behavior. At further lower temperatures of around 40 K theo-Ps state with longer lifetime and larger intensity became overwhelming for all molecules. Also ₃ andI ₃ were not very sensitive to most of the phase transitions, including the melting points.     

Reactions of chlorine dioxide with organic compounds. 2. Oxidation of thiols

The method for the synthesis of 4-aryl-2,6,6-trimethyl-5-oxo-5,6-dihydro-4H-thieno[3,2-b]pyrroles from accessible 4-aminothiophene derivatives was developed.     

Alternative CHCA-based matrices for the analysis of low molecular weight compounds by UV-MALDI-tandem mass spectrometry

Analysis of low molecular weight compounds (LMWC) in complex matrices by vacuum matrix-assisted laser desorption/ionization (MALDI) often suffers from matrix interferences, which can severely degrade limits of quantitation. It is, therefore, useful to have available a range of suitable matrices, which exhibit complementary regions of interference. Two newly synthesized α-cyanocinnamic acid derivatives are reported here; (E)-2-cyano-3-(naphthalen-2-yl)acrylic acid (NpCCA) and (2E)-3-(anthracen-9-yl)-2-cyanoprop-2enoic acid (AnCCA). Along with the commonly used α-cyano-4-hydroxycinnamic acid (CHCA), and the recently developed 4-chloro-α-cyanocinnamic acid (Cl-CCA) matrices, these constitute a chemically similar series of matrices covering a range of molecular weights, and with correspondingly differing ranges of spectral interference. Their performance was compared by measuring the signal-to-noise ratios (S/N) of 47 analytes, mostly pharmaceuticals, with the different matrices using the selected reaction monitoring (SRM) mode on a triple quadrupole instrument equipped with a vacuum MALDI source. AnCCA, NpCCA and Cl-CCA were found to offer better signal-to-noise ratios in SRM mode than CHCA, but Cl-CCA yielded the best results for 60% of the compounds tested. To better understand the relative performance of this matrix series, the proton affinities (PAs) were measured using the kinetic method. Their relative values were: AnCCA > CHCA > NpCCA > Cl-CCA. This ordering is consistent with the performance data. The synthesis of the new matrices is straightforward and they provide (1) tunability of matrix background interfering ions and (2) enhanced analyte response for certain classes of compounds.     

Water-soluble cellulose derivatives prepared by the action of solutions of nitrogen dioxide in dimethylformamide

Cellulose derivatives soluble in water and in dilute alkali were prepared by dissolving a bleached sulfite pulp in solutions of nitrogen dioxide in dimethylformamide (DMF) and heating at 70–90°C for 1–3 hr. No cellulose oxidation was detected. Nitrite esters and apparently also, nitrate esters, were formed with degrees of substitution (DS) ranging from 0.18 to 0.55 and with nitrogen contents ranging from 1.47 to 4.32%. Solution viscosities in water increased with increasing nitrogen content. The degree of polymerization (DP) decreased rapidly in the initial stages of the reaction followed by a slow, gradual decrease with increasing reaction times.     

Formation of low molecular weight carbonyl compounds by sensitized photochemical decomposition of aliphatic hydrocarbons in seawater

Summary Anthraquinone has been found to be a compound occuring frequently in seawater. Excited by solar radiation it acts as one of many natural and man-made photosensitizers and can thus be used as a model substance for the study of sensitized photochemical reactions of environmental chemicals. Experimentally it has been shown to mediate the oxidative photochemical decomposition of aliphatic hydrocarbons which, lacking absorption bands in the solar UV range at sea level, are by themselves photochemically inert. Formaldehyde, smaller amounts of acetaldehyde and acetone as well as a still unidentified carbonyl compound are the principal low molecular weight products generated in the anthraquinone-sensitized photooxidation, with natural as well as artificial sunlight, of straight chain saturated hydrocarbons accommodated in high purity water. Qualitatively the same results were obtained in natural seawater as reaction medium from which particles were removed by glass fiber filtration and organic compounds by adsorption on activated charcoal. The concomittant generation of homologous series of methylketones and terminal alkenes suggest a decomposition mechanism involving cyclic electron rearrangement in a 6-membered transition state. Based on HPLC analysis of their 2,4-dinitrophenylhydrazones, the rates of volatile carbonyl generation in the sensitized photo-oxidation of n-tetradecane was determined in the liquid phase. Also determined was the rate of formaldehyde formation in the gas phase from n-tetradecane and from two Brazilian crude oils. The rates of generation of acetaldehyde and acetone could not be determined in the experiments with crude oils because of irregular changes of concentrations with time. The rate of concentration increase of formaldehyde in the vapour phase over the hydrocarbon surface film was similar to that in the water underneath. The artifical light source was a high pressure xenon lamp whose emission spectrum closely resembles that of natural sunlight at sea level. It was calibrated against the intensity of natural sunlight using pnitroacetophenone/pyridine as binary chemical actinometer.     

Oxidation of thiol compounds by molecular oxygen in aqueous solutions

Side self-oxidation of thiols was studied. It was found that these reactions in neutral and alkaline solutions are induced by impurities of variable-valence metals. The ability of transition metals to catalyze oxidation of thiols changes in the order Cu > Mn > Fe > Ni Co. The plot of the self-oxidation rate vs. pH passes through a maximum whose position on the pH scale depends on both the nature of metal and the structure of the thiol oxidized. For thiols having different structures, the kinetic orders in reactions catalyzed by copper ions differently vary with pH, which is apparently associated with the formation of complexes possessing different catalytic activity.     

Purification and characterization of two low molecular weight endoglucanases produced by Penicillium occitanis mutant pol 6

Two endoglucanases (EGs), EG A and EG B, were purified to homogeneity from Penicillium occitanis mutant Pol 6 culture medium. The molecular weights of EG A and EG B were 31,000 and 28,000 kDa, respectively. The pI was about 3 for EG A and 7.5 for EG B. Optimal activity was obtained at pH 3.5 for both endoglucanases. Optimal temperature for enzyme activity was 60 degrees C for EG A and 50 degrees C for EG B. EG A was thermostable at 60 degrees C and remained active after 1 h at 70 degrees C. EGs hydrolyzed carboxymethylcellulose, phosphoric acid swollen cellulose, and beta-glucan efficiently, whereas microcrystalline cellulose (Avicel) and laminarin were poorly hydrolyzed. Only EG B showed xylanase activity. Furthermore, these EGs were insensitive to the action of glucose and cellobiose but were inhibited by the divalent cations Hg2+, Co2+, and Mn2+.     

Rh-catalyzed carbonylation of arylzinc compounds yielding symmetrical diaryl ketones by the assistance of oxidizing agents

Carbonylative homocoupling of arylzinc compounds 1 using 1 atm of CO and 1,2-dibromoethane as an oxidant was achieved in the presence of Rh-dppf catalyst, affording symmetrical diaryl ketones in good yields. Under similar conditions, Pd or Ni catalysts induced oxidative homocoupling of 1 to yield biaryls instead. The beneficial catalysis by Rh in the carbonylation was presumed to stem from the facility by which the migration of the aryl ligand to CO at the Rh(3+) intermediate occurred.     

Oxidation of alcohols to carbonyl compounds with CrO3.SiO2 in supercritical carbon dioxide

Supercritical carbon dioxide (scCO2) is an effective reaction medium to perform the oxidation of primary and secondary aliphatic alcohols to the corresponding carbonyl compounds with chromium trioxide supported on silica. These reactions were performed by flowing a solution of the alcohol in scCO2 through a column containing the supported reagent and recovering the product by depressurization. This method avoids the use of organic solvents and the contamination of the products with chromium species.     

Approaches for the analysis of low molecular weight compounds with laser desorption/ionization techniques and mass spectrometry

This review summarizes various approaches for the analysis of low molecular weight (LMW) compounds by different laser desorption/ionization mass spectrometry techniques (LDI-MS). It is common to use an agent to assist the ionization, and small molecules are normally difficult to analyze by, e.g., matrix assisted laser desorption/ionization mass spectrometry (MALDI-MS) using the common matrices available today, because the latter are generally small organic compounds themselves. This often results in severe suppression of analyte peaks, or interference of the matrix and analyte signals in the low mass region. However, intrinsic properties of several LDI techniques such as high sensitivity, low sample consumption, high tolerance towards salts and solid particles, and rapid analysis have stimulated scientists to develop methods to circumvent matrix-related issues in the analysis of LMW molecules. Recent developments within this field as well as historical considerations and future prospects are presented in this review.     

Accurate mass measurement of low molecular weight compounds by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

We report the application of matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) for the accurate measurement of mass of low molecular weight compounds (smaller than 1500 Da), a linear peptide, two types of cyclic depsipeptides, a polyhydroxy-macrocyclic lactone, and two prenylated flavonoids, with delayed extraction in the reflector mode. The performance of the MALDI-TOF instrument was less than those of fast atom bombardment and Fourier-transform ion cyclotron resonance mass spectrometry instruments and insufficient to give acceptable accuracy for literature reporting. Nevertheless, when combined with NMR spectrometry and/or amino acid analysis to give information on the numbers of carbon atoms and index of hydrogen deficiency, MALDI was useful for determination of the elemental composition of the low molecular weight compounds available in small quantities.     

Analytical scale supercritical fluid fractionation of a low molecular weight,high density polyethylene wax with carbon dioxide

This paper reports on the use of supercritical carbon dioxide to fractionate a low molecular weight, high density polyethylene into very narrow molecular weight distributions (MWDs). A simple extraction system was developed that allowed relatively trouble free extractions of the polyethylene samples. Fractions were collected at successively higher densities of CO₂ at constant temperature and then analyzed by capillary supercritical fluid chromatography (SFC), or high temperature gel permeation chromatography (GPC) and/or differential scanning calorimetry (DSC). Fractionations were performed at three different temperatures (60,80, and 100°C). Higher temperatures were found to yield greater recoveries and higher MWDs at any given density. Reducing the increment between successive extraction steps resulted in polydispersities being reduced to nearly “monodisperse” levels. Total recoveries ranged from 12 to 33% depending on the temperature, and the highest molecular weight fraction extracted by the CO₂ was centered around 1500.     

Unstable complexes of sandwich compounds with small molecules. Communication 3. Complexation of metallocenes of the 3d series with molecular oxygen at low temperatures

Conclusions Using the IR spectroscopy method we have shown that at low temperatures metallocenes (⁵-C₅H₅)₂M (M=V, Cr, Mn, Fe, Co, Ni) form complexes with O₂ in which the O₂ molecule is coordinated to the metal atom through one of the oxygen atoms, forming a complex of the M-O-O type.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 1, pp. 58–60, January, 1988.     

Non-fouling surfaces produced by gas phase pulsed plasma polymerization of an ultra low molecular weight ethylene oxide containing monomer

The pulsed plasma polymerization of low molecular weight molecules containing only one (ethylene oxide vinyl ether) and two (diethylene oxide vinyl ether) ethylene oxide units were investigated. The surface density of EO units retained in the polymer films increases sharply with decreasing average power input during deposition, particularly at very low plasma duty cycles. The protein adsorption properties of these plasma synthesized polymer were investigated using ¹²⁵I-labeled albumin and fibrinogen. Surprisingly effective, non-fouling surfaces were observed with films synthesized from the monomer containing two ethylene oxide units; however, the monomer containing only one EO unit gave surfaces that were not particularly effective in preventing protein adsorptions. The results obtained show that ultra short chain length PEO modified surfaces can be biologically non-fouling. This, in turn, has interesting consequences in terms of trying to identify the basic reason for the effectiveness of EO units in preventing biomolecule adsorptions on surfaces.     

Superheated water chromatography of low molecular weight polyethylene glycols with ultraviolet detection.

Superheated water chromatography (SWC) with ultraviolet detection was applied to the separation of low molecular weight polyethylene glycols (PEGs). PEG oligomers could be detected sensitively when the detection wavelength was set at 190 nm. The effect of column temperature on the separation of PEG oligomers was investigated. The elution time of all PEG oligomers decreased with increase in the column temperature; linear relationships were obtained between ln k and 1/T. A temperature-programmed SWC separation enabled the baseline separation of a PEG 200 sample within 50 min.