Sensitive determination of a glyoxal-DNA adduct biomarker candidate by column switching capillary liquid chromatography electrospray ionization mass spectrometry

A method based on column switching packed capillary liquid chromatography electrospray mass spectrometry has been developed for the determination of the adduct glyoxal-deoxyguanosine, a biomarker candidate for the assessment of glyoxal exposure, in DNA hydrolysate solutions. Microgram amounts of DNA were isolated and enzymatically hydrolyzed to deoxyribonucleosides, prior to ultrafiltration and subsequent dilution to a sample solution consisting of water-acetonitrile-formic acid (98 : 2 : 0.2, v/v). The sample solution was loaded onto a 1 mm I.D. x 5 mm Hypercarb (5 mum) porous graphitic carbon trap column for analyte enrichment using an injection volume of 200 mul, and was subsequently back-flushed onto a 0.30 mm I.D. x 150 mm Lichrospher diol (5 mum) analytical column. The samples were loaded with a flow rate of 40 mul min(-1) and glyoxal-deoxyguanosine was desorbed from the trap column and eluted with an isocratic mobile phase consisting of water-acetonitrile-formic acid (50 : 50 : 0.2, v/v) at a flow rate of 5 mul min(-1). Mass spectrometric determination of glyoxal-deoxyguanosine was obtained by multiple reaction monitoring of the transition [M + H](+)m/z 326 --> m/z 210. The method was evaluated over the concentration range 0.25-50 ng ml(-1) of glyoxal-deoxyguanosine in the hydrolysate of 5 mug DNA. The method was linear with a correlation coefficient of 0.9998 in this range. The within-day (n = 6) and between-day (n = 6) precisions were determined as 1.2-11% and 1.4-11% RSD, respectively, and the recovery was close to 100%. The mass limit of detection was 15 pg, corresponding to a concentration limit of detection of 75 fg mul(-1) DNA hydrolysate solution, corresponding to 48 adducts per 10(6) normal nucleosides. The method was applied for the determination of glyoxal-deoxyguanosine in DNA hydrolysate solutions of calf thymus DNA and cell cultures after reaction or incubation with glyoxal.     

Mechanism of activation of a hafnium pyridyl-amide olefin polymerization catalyst: ligand modification by monomer

We have investigated the olefin polymerization mechanism of hafnium catalysts supported by a pyridyl-amide ligand with an ortho-metalated naphthyl group. Ethylene-alpha-olefin copolymers from these catalysts have broad molecular weight distributions that can be fit to a bimodal distribution. We propose a unique mechanism to explain this behavior involving monomer modification of the catalyst, which generates multiple catalyst species when multiple monomers are present. More specifically, we present evidence that the hafnium alkyl cation initially undergoes monomer insertion into the Hf-naphthyl bond, which permanently modifies the ligand to generate new highly active olefin polymerization catalysts. Under ethylene/octene copolymerization conditions, a plurality of new catalysts is formed in relative proportion to the respective monomer concentrations. Due to the asymmetry of the metal complex, two "ethylene-inserted" and eight "octene-inserted" isomers are possible, but it is a useful approximation to consider only one of each in the polymerization behavior. Consequently, gel permeation chromatography data for the polymers can be fit to a bimodal distribution having a continuous shift from a predominantly low molecular weight fraction to predominantly higher molecular weight fraction as [octene]/[ethylene] is increased. Theoretical calculations show that such insertions into the Hf-aryl bond have lower barriers than corresponding insertions into the Hf-alkyl bond. The driving forces for this insertion into the Hf-aryl bond include elimination of an eclipsing H-H interaction and formation of a stabilizing Hf-arene interaction. These new "monomer-inserted catalysts" have no beta-agostic interaction, very weak olefin binding, and olefin-insertion transition states which differ on the two sides by more than 4 kcal/mol. Thus, the barrier to site epimerization is very low and high polymerization rates are possible even when the chain wags prior to every insertion. Experimental evidence for aryl-insertion products is obtained from reactions of ethylene (13C2H4 NMR studies) or 4-methyl-1-pentene (4M1P) using relatively low monomer/catalyst ratios. Quantitative generation of monomer-inserted products is complicated by slow initiation kinetics followed by fast polymerization kinetics. However, NMR evidence for reaction with 13C2H4 was observed in situ at low temperature, and the attachment of monomer to ligand was confirmed by GC/MS and 13C NMR after quenching. Furthermore, a 4M1P-appended ligand was isolated from a polymerization reaction (50:1 monomer:catalyst) by column chromatography followed by multiple recrystallizations. One isomer was characterized by X-ray crystallography, which unequivocally shows a 4-methylpentyl substituent at the 2-position of the naphthyl, consistent with 1,2-insertion into the Hf-aryl bond. NMR suggests a second diastereomer (not isolated) is formed from a 1,2-insertion of opposite stereoselectivity.     

ANALYSIS OF A COMPRESSIBLE GAS-LIQUID MODEL MOTIVATED BY OIL WELL CONTROL OPERATIONS Dedicated to Professor Constantine M. Dafermos on the occasion of his 70th birthday

We are interested in a viscous two-phase gas-liquid mixture model relevant for modeling of well control operations within the petroleum industry.We focus on a simplified mixture model and provide an existence result within an appropriate class of weak solutions.We demonstrate that upper and lower limits can be obtained for the gas and liquid masses which ensure that transition to single-phase regions do not occur.This is used together with appropriate a prior estimates to obtain convergence to a weak solution for a sequence of approximate solutions corresponding to mollified initial data.Moreover,by imposing an additional regularity condition on the initial masses,a uniqueness result is obtained.The framework herein seems useful for further investigations of more realistic versions of the gas-liquid model that take into account different flow regimes.     

Global weak solutions for a compressible gas-liquid model with well-formation interaction

The objective of this work is to explore a compressible gas-liquid model designed for modeling of well flow processes. We build into the model well-reservoir interaction by allowing flow of gas between well and formation (surrounding reservoir). Inflow of gas and subsequent expansion of gas as it ascends towards the top of the well (a so-called gas kick) represents a major concern for various well operations in the context of petroleum engineering. We obtain a global existence result under suitable assumptions on the regularity of initial data and the rate function that controls the flow of gas between well and formation. Uniqueness is also obtained by imposing more regularity on the initial data. The key estimates are to obtain appropriate lower and upper bounds on the gas and liquid masses. For that purpose we introduce a transformed version of the original model that is highly convenient for analysis of the original model. In particular, in the analysis of the transformed model additional terms, representing well-formation interaction, can be treated by natural extensions of arguments that previously have been employed for the single-phase Navier-Stokes model. The analysis ensures that transition to single-phase regions do not appear when the initial state is a true gas-liquid mixture.     

RECENT DEVELOPMENTS ON THE STRUCTURAL CHEMISTRY OF COMPLEXES OF SELENIUM AND TELLURIUM WITH SULFUR-AND SELENIUM CONTAINING LIGANDS

Abstract

The stereochemistry of selenium and tellurium and the role of lone electron pairs will be discussed on the basis of recent X-ray diffraction studies appearing in the literature. Some of the studies represent new types of structures.     

Global weak solutions for a viscous liquid–gas model with transition to single-phase gas flow and vacuum

This work deals with a viscous two-phase liquid–gas model relevant to the flow in wells and pipelines. The liquid is treated as an incompressible fluid whereas the gas is assumed to be polytropic. The model is rewritten in terms of Lagrangian coordinates and is studied in a free boundary setting where the liquid and gas masses are of compact support initially, and continuous at the boundary. Consequently, the initial masses involve a transition to single-phase gas flow and vacuum at the boundary. An appropriate balance between pressure and viscous forces is identified which allows obtaining pointwise upper and lower estimates of masses. These estimates rely on the assumption of a certain relation between the rate of degeneracy of the viscosity coefficient and the rate that determines how fast the initial masses are vanishing at the boundary. By combining these estimates with basic energy type of estimates, higher order regularity estimates are obtained. The existence of global weak solutions is then proved by showing compactness for a class of semi-discrete approximations.     

Formation of the porous structure during the polymerization of meta-divinylbenzene and para-divinylbenzene with toluene and 2-ethylhexanoic acid (2-EHA) as porogens

Macroporous polymers of pure meta-divinylbenzene (meta-DVB) and pure para-divinylbenzene (para-DVB) have been prepared in the presence of toluene and 2-EHA as pore forming agents. The formation of the pore structure has been studied during the polymerization by pore-size distribution measurements, together with determination of the specific surface area from nitrogen sorption isotherms using the BET treatment. In addition, the morphology and texture have been characterized by SEM during the polymerization process. Large differences in the pore-size distribution among all the polymer samples are found. The polymers prepared in toluene as porogen have a pore-size distribution, which mainly consists of small pores, while large pores appear with 2-EHA as porogen. In the presence of 2-EHA, a major change in the pore-size distribution is also observed when the monomer is shifted from para-DVB to meta-DVB, leading to a bimodal distribution. The texture characterization by SEM shows details and discriminates the samples in consistency with what may be expected from pore-size distribution measurements. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 3973–3990, 1999     

Well-Posedness of A Compressible Gas-Liquid Model for Deepwater Oil Well Operations

The main purpose of this paper is two-fold: (i) to generalize an existence result for a compressible gas-liquid model with a friction term recently published by Friis and Evje [SIAM J. Appl. Math., 71 (2011), pp. 2014–2047]; (ii) to derive a uniqueness result for the same model. A main ingredient in the existence part is the observation that we can consider weaker assumptions on the initial liquid and gas mass, and still obtain an existence result. Compared to the above mentioned work, we rely on a more refined application of the estimates provided by the basic energy estimate. Concerning the uniqueness result, we borrow ideas from Fang and Zhang [Nonlinear Anal. TMA, 58 (2004), pp. 719–731] and derive a stability result under appropriate constraints on parameters that determine rate of decay toward zero at the boundary for gas and liquid masses, and growth rate of masses associated with the friction term and viscous coefficient.     

Discrete approximations of BV solutions to doubly nonlinear degenerate parabolic equations

Summary. In this paper we present and analyse certain discrete approximations of solutions to scalar, doubly nonlinear degenerate, parabolic problems of the form under the very general structural condition . To mention only a few examples: the heat equation, the porous medium equation, the two-phase flow equation, hyperbolic conservation laws and equations arising from the theory of non-Newtonian fluids are all special cases of (P). Since the diffusion terms a(s) and b(s) are allowed to degenerate on intervals, shock waves will in general appear in the solutions of (P). Furthermore, weak solutions are not uniquely determined by their data. For these reasons we work within the framework of weak solutions that are of bounded variation (in space and time) and, in addition, satisfy an entropy condition. The well-posedness of the Cauchy problem (P) in this class of so-called BV entropy weak solutions follows from a work of Yin [18]. The discrete approximations are shown to converge to the unique BV entropy weak solution of (P). Received November 10, 1998 / Revised version received June 10, 1999 / Published online June 8, 2000