Distribution Of Oligomers Research Articles

Highly Efficient Zirconium-Catalyzed Batch Conversion of 1-Butene: A New Route to Jet Fuels

Quantitative conversion of 1-butene to a Schultz−Flory distribution of oligomers has been accomplished by use of Group 4 transition-metal catalysts in the presence of methylaluminoxane (MAO). The o...

Crystal structures of the effector‐binding domain of repressor Central glycolytic gene Regulator from Bacillus subtilis reveal ligand‐induced structural changes upon binding of several glycolytic intermediates

Expression of genes in the gapA operon encoding five enzymes for triose phosphate interconversion in Bacillus subtilis is negatively regulated by the Central glycolytic genes Regulator (CggR). CggR belongs to the large SorC/DeoR family of prokaryotic transcriptional regulators, characterized by an N-terminal DNA-binding domain and a large C-terminal effector-binding domain. When no glucose is present in growth media, CggR binds to its target DNA sequence and blocks the transcription of genes in the gapA operon. In the presence of glucose, binding of the known effector molecule fructose-1,6-bisphosphate abolishes this interaction. We have identified dihydroxyacetone phosphate, glucose-6-phosphate and fructose-6-phosphate as additional CggR ligands that can bind to the effector-binding site. Crystal structures of C-CggR, the C-terminal effector-binding domain of CggR, both unliganded as well as in complex with the four ligands at resolutions between 1.65 and 1.80 A reveal unique ligand-specific structural changes in the binding site that affect the dimer interface. Binding affinities of these ligands were determined by isothermal titration calorimetry. Chemical cross-linking shows that CggR oligomerization is mediated through its effector-binding domain, and that binding of the different ligands differentially affects the distribution of oligomers. Electrophoretic mobility shift assays (EMSAs) confirmed a destabilizing effect of fructose-1,6-bisphosphate on the CggR/DNA complex, and also showed similar effects for dihydroxyacetone phosphate. Our results suggest that CggR stability and function may be modulated by various effectors in a complex fashion.

High resolution NMR for the direct characterisation of complex polyoxyethylated alcohols (C iE j) mixtures

High resolution 1H and 13C NMR were used to characterise complex mixtures of industrial polyoxyethylated surfactants of the C i E j series. A progressive methodology was applied to analyse mixtures of monodisperse surfactants of increasing complexity, with regard to both the alkyl chain ( i = 8–16) and the polyoxyethylene moiety ( j = 1–8). The techniques were then successfully applied for the characterisation of narrow- and broad-range commercial surfactants and to analyse the widely used Brij 30. It appears that direct high resolution NMR analysis of ethoxylated alcohol mixtures offers simultaneously a good description of the alkyl chain composition as regards to the length distribution and branching, together with an accurate overview of the oxyethylene oligomeric distribution for ethoxylates containing up to eight units. Finally NMR allows to check free alcohol and free polyethylene oxide that may exist in commercial surfactants.

Controlling the Oligomeric Composition of Carbon‐Fiber Precursors by Dense‐Gas Extraction

Dense‐gas extraction of M‐50 petroleum pitch with the solvent toluene was carried out in a continuous, countercurrent‐flow, packed column at temperatures from 330° to 380°C, pressures from 42 to 84 bar, and a solvent‐to‐pitch ratio of 5:1. Both one‐ and two‐column configurations were used to fractionate raw pitches into top and bottom cuts of controlled oligomeric distribution. Using matrix‐assisted laser desorption–ionization mass spectroscopy for absolute molecular weight determination, the effect of changes in pitch oligomeric distribution on both the softening point and the degree of anisotropy (i.e., isotropic–mesophase equilibrium) was established. For example, pitch fractions rich in dimer and trimer species, and with essentially all monomer removed, exhibit the anisotropic microstructure typically found in precursors for high‐performance carbon fibers.

Biodegradability of highly ethoxylated nonionic surfactants: Determination of intermediates and pathways of biodegradation

Existing data regarding alcohol ethoxylate (AE) surfactants indicate that structures with greater than 20 ethoxylate (EO) units per molecule or with multibranched alkyl chains may not pass a ready biodegradability test. This could have important consequences for complying with regional regulatory requirements and for the potential risks these chemicals could present to the environment. We investigated the influence of chemical structure on the biodegradability of AEs with different alkyl chain branching and EO content. The AEs investigated were a multibranched AE (average, 18 EO), an oxo-AE (monobranched; average, 23 EO), and a linear AE of oleochemical origin (average, 40 EO). The aims of the present study were to assess the ready biodegradability of AEs with high EO content and to establish the mechanism or pathway by which biodegradation occurs for the oxo-AE. Biodegradation studies were conducted using standard test conditions (International Standards Organization 14593). Solid-phase extraction and liquid chromatography with electrospray mass spectrometry were used to detect profiles in both derivatized and underivatized extracts of samples. Derivatization with phthalic anhydride was used to improve ionization of the lower-ethoxylated AEs and free alcohol that were key indicators in the present study. All AEs were rapidly biodegraded, achieving more than 60% mineralization of parent material. Central fission was the predominant mechanism for the oxo-AE, as confirmed by identification of the oligomeric distribution and quantification of polyethylene glycol released during biodegradation.

Simultaneous monitoring of peptide aggregate distributions, structure, and kinetics using amide hydrogen exchange: Application to Aβ(1‐40) fibrillogenesis

Increasing evidence indicates that soluble aggregates of amyloid beta protein (Abeta) are neurotoxic. However, difficulty in isolating these unstable, dynamic species impedes studies of Abeta and other aggregating peptides and proteins. In this study, hydrogen-deuterium exchange (HX) detected by mass spectrometry (MS) was used to measure Abeta(1-40) aggregate distributions without purification or modification that might alter the aggregate structure or distribution. Different peaks in the mass spectra were assigned to monomer, low molecular weight oligomer, intermediate, and fibril based on HX labeling behavior and complementary assays. After 1 h labeling, the intermediates incorporated approximately ten more deuterons relative to fibrils, indicating a more solvent exposed structure of such intermediates. HX-MS also showed that the intermediate species dissociated much more slowly to monomer than did the very low molecular weight oligomers that were formed at very early times in Abeta aggregation. Atomic force microscopy (AFM) measurements revealed the intermediates were roughly spherical with relatively homogenous diameters of 30-50 nm. Quantitative analysis of the HX mass spectra showed that the amount of intermediate species was correlated with Abeta toxicity patterns reported in a previous study under the same conditions. This study also demonstrates the potential of the HX-MS approach to characterizing complex, multi-component oligomer distributions of aggregating peptides and proteins.

Degradation of copolymers obtained by ring-opening polymerization of glycolide and ɛ-caprolactone: A high resolution NMR and ESI-MS study

Electrospray ionization-mass spectrometry (ESI-MS) and proton nuclear magnetic resonance ( 1H NMR) have been used to investigate the hydrolytic degradation of copolymers obtained by bulk ring-opening copolymerization of glycolide and ɛ-caprolactone with monomer ratios ranging from 70/30 to 30/70. NMR allows changes of the average sequence distribution and composition of the components to be followed. In contrast, ESI-MS is able to reveal the detailed chemical structures of various sequences despite the molecular weight limit of 2000 Da. Combination of ESI-MS with NMR can thus provide information to describe microstructure changes during degradation. The distribution of various oligomers shown in the form of planar projections is of great interest for the design of biodegradable system aimed at medical applications.

Solution NMR Characterizations of Oligomerization and Dynamics of Equine Infectious Anemia Virus Matrix Protein and Its Interaction with PIP2

Budding of retroviruses requires the structural precursor polyprotein, Gag, to target the plasma membrane through its N-terminal matrix (MA) domain. For HIV-1, the interaction between membrane signaling molecule phosphatidylinositol 4,5-diphosphate (PIP2) and MA induces the exposure of myristate and promotes membrane binding. Here we studied oligomerization of the naturally unmyristylated equine infectious anemia virus (EIAV) MA and its interaction with PIP2-C4 primarily using solution NMR spectroscopy. The measured 1H-15N residual dipolar coupling agrees with the atomic coordinates from the EIAV MA crystal structure. The analytical ultracentrifugation results show a dominant population of monomeric EIAV MA at a concentration of 63 microM and 20 degrees C, along with a small trimer and a broad distribution of other oligomers. The monomer-trimer equilibrium model and the quaternary packing of the trimer were further established by the concentration-dependent 15N spin relaxation rates and chemical shifts. Binding of MA to PIP2-C4 was detected by chemical shift mapping (CSM) with an apparent Kd of 182 +/- 56 microM, a value similar to that reported for HIV-1 MA. The PIP2 binding site includes the Loop region between Helix2 and Helix3 in the EIAV MA. CSM and spin relaxation dispersion reveal a coupling of conformational change and submillisecond dynamics, respectively, between the Loop and trimeric Interface Residues due to PIP2 binding. We infer that PIP2 participates in the initial trimer formation of EIAV MA, but more importantly, the concentration effect is dominant in shifting the equilibrium toward trimer, in line with the entropic switch mechanism proposed for myristylated HIV-1 MA.

Colloidal surfaces and oligomeric species generated by water treatment chemicals

Properties of water treatment chemicals were investigated by a novel combination of analytical methods. Oligomer distribution, total surface charge, zeta potential and conductivity were evaluated in simulated systems containing highly and moderately basic polyaluminium chlorides, aluminium sulfate (alum), ferric chloride and ferric sulfate. Species were quantified on the basis of mass spectro-metric results. Polyaluminium chemicals gave rise to an intensive and completely cationic distribution of mainly Al13-based oligomers and high surface charges. Both cationic distribution and charge decreased with increase of pH and increased with concentration of the chemical. The other chemicals produced a wider oligomeric distribution of smaller species, including anions. The dimeric form was typical for iron. In the case of the sulfate-containing chemicals, sulfate was present in the majority of oligomers and zeta potentials were stable. For the chemicals without sulfate, pH values needed to be high enough to provide high zeta values. The results can be utilized in the improvement of coagulation and flocculation mechanisms aimed at the removal of bacteria, viruses and other micro organisms, as well as at humic and mineral particles. In addition, semi quantitative information about oligomeric species may be helpful in breaking down cell membranes to support disinfection.

Synthesis, Characterization, and Ethylene Oligomerization and Polymerization by 2-Quinoxalinyl-6-iminopyridine Chromium Chlorides

A series of chromium(iii) complexes ligated by N^N^N tridentate 2-quinoxalinyl-6-iminopyridine were synthesized and characterized by elemental analysis and infrared spectroscopy. Single-crystal X-ray diffraction analysis for the structure of complex C3 reveals a distorted octahedral geometry. When methylaluminoxane was employed as the co-catalyst, the chromium complexes showed high activities for ethylene oligomerization and polymerization. The distribution of oligomers obtained followed Schulz–Flory rules with high selectivity for α-olefins. Both steric and electronic effects of coordinated ligands affected the catalytic activities as well as the properties of the catalytic products. The parameters of the reaction conditions were also investigated to explore the optimum catalytic potentials of these complexes.

New tripodal iminophosphorane-based ethylene oligomerization catalysts: Part II. Catalytic behavior

Seven transition-metal complexes of general formula RC ( C H 2 NP ⁡ R ′ 3 ) 3 ) M X 2 based on new tripodal iminophosphorane ligands were investigated as initiators for the oligomerization of ethylene in the presence of aluminum co-catalysts using high-throughput techniques. In all cases, ethylene consumption peaked at ca. 30 °C and was not drastically affected by varying the nature of the metal (M = Ni, Fe, Pd, Cu), the aluminum co-catalyst (MMAO, Et 2AlCl, or EtAlCl 2) or the substituents of the tris(iminophosphorane) ligand (R = Me, Ph; R′ = cyclopentyl, Ph). Structural modifications of the organometallic complexes, either at the metal center or within the tripodal ligand, however, had a significant impact on the oligomer distribution obtained. In particular, Pd-based catalyst (PhC(CH 2NPPh 3) 3)PdCl 2 displayed an excellent selectivity toward hexene formation.

Characterization of the degree of polymerization of xylooligomers produced by flowthrough hydrolysis of pure xylan and corn stover with water

Mechanisms that control xylan removal during pretreatment of lignocellulosic biomass are not well understood. For example, although hemicellulose hydrolysis is virtually always assumed to follow first-order homogeneous kinetics, the increase in xylan removal with flow rate for flowthrough pretreatment systems is inconsistent with the predictions for such models, and better information is needed to understand the causes of such discrepancies. Thus, new methods were developed to follow the fate of xylooligomers with degrees of polymerization of up to 30, a range not possible before, for water-only flowthrough pretreatment of oat spelt xylan and corn stover for temperatures of 200–240 °C. Material balances based on the oligomer release profiles produced by batch and flowthrough operations could be closed, suggesting the methods were quite accurate. However, the results also showed that increasing the flow rate from 0 to 2 and then 25 mL/min affected the size distribution of the xylan oligomers (DP < 30) released from corn stover but not from oat spelt xylan and also increased overall hemicellulose sugar solubilization. One explanation for these difference is that lignin and lignin–xylan compounds in particular play an important role in the hydrolysis of lignocellulosic biomass.

Novel initiators for the synthesis of propylene oxide oligomers by anionic ring opening polymerization

A series of potassium alcoholates was obtained from the reaction between KOH and ethylene glycol, resorcinol, 4,4′-bisphenol A, 4,4′-(1,3 phenylenediisopropylidene)-bisphenol, 4,4′-sulfonyldiphenol. These salts were employed to initiate the anionic ring opening polymerization of propylene oxide (PO). The molecular weight distribution of the propylene oxide oligomers prepared by this method and the initiator structure were correlated. These oligomers were characterized through Fourier transform infrared (FTIR), nuclear magnetic resonance (NMR) spectroscopies and size exclusion chromatography (SEC). It was found that the molecular weight and polydispersity of the synthesized poly(propylene oxide) (PPO) is highly dependant on the initiator structure and solubility in the reactive medium. The oligomers obtained using di-potassium resorcinolate exhibited a molecular weight distribution more polydisperse than that of PO oligomers synthesized by means of di-potassium ethylene glycolate. In the case of the PO polymerizations started by the potassium salts of 4,4′-bisphenol A, 4,4′-(1,3 phenylenediisopropylidene)-bisphenol and 4,4′-sulfonyldiphenol, the oligomer chains showed very broad molecular weight distributions. In general, lower solubility and augmentation of the polymer polydispersity were observed when the number of aromatic rings in the initiator structure increased. The experimental results were contrasted with those obtained from quantum chemical semiempirical calculations at AM1 level. The peculiar behavior exhibited by the initiators with an aromatic structure could be explained in terms of different reactivities of the initiation sites. The theoretical studies revealed that the ring in the aromatic initiators promotes an unsymmetrical growing when the PPO chains are formed. In contrast, the identical reactivity of both initiation sites in the ethylene glycolate produces a symmetrical growing during the PO polymerization.

Chromium(III) complexes ligated by 2-(1-isopropyl-2-benzimidazolyl)-6-(1-(arylimino)ethyl)pyridines: Synthesis, characterization and their ethylene oligomerization and polymerization

A series of chromium(III) complexes bearing 2-(1-isopropyl-2-benzimidazolyl)-6-(1-(arylimino)ethyl)pyridines were synthesized and characterized by IR spectroscopic and elemental analysis. The X-ray crystallographic analysis revealed a distorted octahedral geometry of the chromium complexes. When activated by Et 2AlCl, MAO or MMAO, these chromium complexes exhibited catalytic activities for ethylene oligomerization and polymerization; while the good to high activities (up to 3.95 × 10 6 g mol −1 (Cr) h −1) were observed in the catalytic systems with MMAO. Therefore, various reaction parameters of the catalytic system with MMAO were investigated in detail. The steric and electronic effects of ligands affected the catalytic activities and the distribution of the products predominantly. Interestingly, sometimes their distributions of oligomers did not resemble the rules of Schulz-Flory or Poisson due to the hexenes produced in low yield.

Resolving Oligomers from Fully Grown Polymers with IMS−MS

Ion mobility and mass spectrometry techniques, combined with electrospray ionization, have been used to examine distributions of poly(ethylene glycols) (PEG) with average molecular masses of 6550 and 17900 Da. The analysis provides information about the polymer size distributions as well as smaller oligomers existing over a wide range of charge states and sizes (i.e., [HO(CH2CH2O)xH + nCs]n+, where x ranges from 21 to 151 and n = 2 to 11 for the 6550 Da sample; and, x ranges from 21 to 362 and n = 2 to 23 for the 17 900 Da sample). The present data show that oligomer distributions also fall into families, corresponding to much narrower size distributions for individual charge states; this dramatically simplifies data analysis. For example, we show evidence for baseline resolution of the +10 charge state of polymers. Unlike the charge-state trends reported previously for peptide ion families, which show generally increasing mobilities with increasing charge state (for a given m/z value), the mobilities of [HO(CH2CH2O)xH + nCs]n+ families generally decrease with increasing charge state. This requires that the addition of charges leads to substantial changes in the average structures of the ions. Comparisons of cross section calculations from molecular modeling results for multiply cesiated PEG ions with experimental cross sections indicate that these ions adopt highly extended (in many cases nearly linear) conformations, except for the high degree of coordination of the charged sites.

Determination of Alkylphenol Ethoxylates in Textiles by Normal-Phase Liquid Chromatography and Reversed-Phase Liquid Chromatography/Electrospray Mass Spectrometry

Comprehensive analytical methods based on pressurized liquid extraction followed by normal-phase liquid chromatography (NPLC) with ultraviolet detection and reversed-phase liquid chromatography (RPLC)/electrospray mass spectrometry (MS) have been developed for determination of alkylphenol ethoxylates (APEOs) in textile samples. NPLC with an aminosilica column allowed for the chromatographic separation of APEOs according to the increasing number of ethylene units and revealed the exact distribution of individual oligomers. RPLC coupled with electrospray MS was highly sensitive and enabled the complete qualitative and quantitative determination of individual APEOs in textile samples. The 2 analytical methods based on different chromatographic separation mechanisms, i.e., NPLC and RPLC, may provide complementary information of APEOs in textile materials. The 2 detection methods were successfully applied to the investigation of various textile samples, and the data of our research suggested actual pollution in real textile products.

New Insight into the Role of the Metal Oxidation State in Controlling the Selectivity of the Cr-(SNS) Ethylene Trimerization Catalyst

The tri- and divalent complexes of the 2,6-bis(RSCH2)pyridine [R = Ph, Cy] ligands have been prepared. Upon activation with MAO, both species are catalysts for ethylene oligomerization of moderate activity. However, while the trivalent catalysts produced only 1-hexene, the divalent species gave a statistical distribution of oligomers. This clear difference in catalytic behavior indicates that the two oxidation states are not interconnected during the catalytic cycle as it happens instead with other oligomerization catalytic systems. The trivalent precursor is not reduced and the divalent is not oxidized. Treatment of the trivalent catalyst precursors with either MAO or other R3Al species afforded intractable materials. Instead, similar reactions with the divalent complexes gave new cationic species, which have been characterized by X-ray analysis. These complexes have preserved the divalent state of chromium during the reaction and still produce, upon further activation, a statistical distribution of olig...

Chromium(III) complexes bearing 2-imino-1,10-phenanthrolines: Synthesis, molecular structures and ethylene oligomerization and polymerization

A series of chromium(III) complexes ligated by tridentate ligands of 2-imino-1,10-phenanthrolines, LCrCl 3 (L = 2-(2,6-(R 1) 2C 6H 3N = CR)-1,10-phen) was synthesized and characterized by elemental and spectroscopic analyses as well as single-crystal X-ray crystallography. X-ray crystallographic analysis of 2 (R = Ph, R 1 = Et), 4 (R = Me, R 1 = Me) and 12 (R = H, R 1 = i-Pr) reveals that the geometry around the chromium atom is octahedral. Upon activation with MAO, these complexes exhibited high activities for ethylene oligomerization (up to 1.15 × 10 7 g mol −1 (Cr) h −1) and moderate activities for ethylene polymerization. The oligomers and polymers obtained were α-olefins, and the distribution of oligomers resembled the Schultz–Flory rule. Various reaction parameters were investigated in detail, and the results revealed that both the steric and electric effects of ligands affected the catalytic activities of their chromium complexes as well as the distribution of the products.

Compositional Analysis of Water-Soluble Materials in Corn Stover

Corn stover is one of the leading feedstock candidates for commodity-scale biomass-to-ethanol processing. The composition of water-soluble materials in corn stover has been determined with greater than 90% mass closure in four of five representative samples. The mass percentage of water-soluble materials in tested stover samples varied from 14 to 27% on a dry weight basis. Over 30 previously unknown constituents of aqueous extracts were identified and quantified using a variety of chromatographic techniques. Monomeric sugars (primarily glucose and fructose) were found to be the predominant water-soluble components of corn stover, accounting for 30-46% of the dry weight of extractives (4-12% of the dry weight of feedstocks). Additional constituents contributing to the mass balance for extractives included various alditols (3-7%), aliphatic acids (7-21%), inorganic ions (10-18%), oligomeric sugars (4-12%), and a distribution of oligomers tentatively identified as being derived from phenolic glycosides (10-18%).

Enhanced adiponectin multimer ratio and skeletal muscle adiponectin receptor expression following exercise training and diet in older insulin-resistant adults

Circulating adiponectin is reduced in disorders associated with insulin resistance. This study was conducted to determine whether an exercise/diet intervention would alter adiponectin multimer distribution and adiponectin receptor expression in skeletal muscle. Impaired glucose-tolerant older (>60 yr) obese (BMI 30-40 kg/m(2)) men (n = 7) and women (n = 14) were randomly assigned to 12 wk of supervised aerobic exercise combined with either a hypocaloric (ExHypo, approximately 500 kcal reduction, n = 11) or eucaloric diet (ExEu, n = 10). Insulin sensitivity was determined by the euglycemic (5.0 mM) hyperinsulinemic (40 mU x m(-2) x min(-1)) clamp. Adiponectin multimers [high (HMW), middle (MMW), and low molecular weight (LMW)] were measured by nondenaturing Western blot analysis. Relative quantification of adiponectin receptor expression through RT-PCR was determined from skeletal muscle biopsy samples. Greater weight loss occurred in ExHypo compared with ExEu subjects (8.0 +/- 0.6 vs. 3.2 +/- 0.6%, P < 0.0001). Insulin sensitivity improved postintervention in both groups (ExHypo: 2.5 +/- 0.3 vs. 4.4 +/- 0.5 mg x kg FFM(-1) x min(-1), and ExEu: 2.9 +/- 0.4 vs. 4.1 +/- 0.4 mg x kg FFM(-1) x min(-1), P < 0.0001). Comparison of multimer isoforms revealed a decreased percentage in MMW relative to HMW and LMW (P < 0.03). The adiponectin SA ratio (HMW/total) was increased following both interventions (P < 0.05) and correlated with the percent change in insulin sensitivity (P < 0.03). Postintervention adiponectin receptor mRNA expression was also significantly increased (AdipoR1 P < 0.03, AdipoR2 P < 0.02). These data suggest that part of the improvement in insulin sensitivity following exercise and diet may be due to changes in the adiponectin oligomeric distribution and enhanced membrane receptor expression.