Acetylation of DMSO:PF solutions of cellulose

Three different techniques for esterifying solutions of cellulose dissolved in mixtures of dimethyl sulfoxide (DMSO) and paraformaldehyde (PF) were evaluated and are herein described. The evaluation and development of suitable synthetic procedures and the characteristics of the resulting acetylated cellulose are reported. Glacial acetic acid (glacial HOAc), acetyl chloride (AcCl), and acetic anhydride (Ac₂O) were compared as acetylating agents for solutions of cellulose in DMSO:PF, and it was demonstrated that mixtures of pyridine (Py) and Ac₂O rapidly acetylated the cellulose to yield beige to amber acetone-soluble cellulose acetates which were partially oxidized. These thermoplastic resins exhibited softening points between 80 and 110°C and thermal stabilities (in nitrogen atmospheres) similar to those of native celluloses (350 to 375°C). Degree of acetyl substitution (DS) values ranged from 0 to 2.0 as a function of acetylation time.     

Mass spectrometry of 1 phenyl-1, 2-dibromopropylphosphonic acid and its methyl ester

$\text{erythro}$-1-Phenyl-1,2-dibromopropylphosphonic acid is known to decompose rapidly into the monomeric metaphospbate-anion (PO₃), 1-phenyl-1-bromopropene, and bromide ion in protic or aprotic solvents in the presence of a hindered tertiary amine. The present study compares the behavior of this phosphonic acid in solution and in the gas phase. The gas-phase thermolysis of the acid involves mainly the loss of bromine and of the phosphoryl group, -P(O)(OR) (OR') (R = R' = H). The same type of pathway is followed in the ionization/excitation processes (“cationic reactions”) that follow from electron impact in mass spectrometry. The thermal and cationic reactions of the monomethyl and dimethyl esters of the phosphoric acids (R=H,R'= CH₃ and R = R' = CH₃, respectively, in -P(O)(OR)(OR')) are entirely analogous to those of the free dibasic acid.     

Preparation of block copolymers from occluded vinyl acetate macroradicals

Stable vinyl acetate macroradicals were produced by polymerization in a nonviscous poor solvent, a viscous good solvent and a viscous poor solvent. These macroradicals were then allowed to react with a second vinyl monomer to produce block copolymers. The formation of block copolymers was monitored for rate and yield data. The block copolymers produced were poly(vinyl acetate-b-methyl methacrylate), poly(vinyl acetate-b-acrylic acid), poly(vinyl acetate-b-vinylpyrrolidone), poly(vinyl acetate-b-acrylonitrile), poly(vinyl acetate-b-styrene), and poly(vinyl acetate-b-methyl acrylate). The block copolymers were characterized by yield, precipitation in selected solvents, pyrolysis gas chromatography, and differential scanning calorimetry.     

SPECTRAL PROPERTIES OF CHLOROPHYLL a MONOLAYERS: MONOLAYERS OF CHLOROPHYLL a AND PHEOPHYTIN AT A GAS-WATER INTERFACE*

Abstract— Surface and spectral properties of chlorophyll a monolayers were studied at a nitrogen-water interface. Direct spectral analysis of Chl monolayers indicated that compression results in a heterogenous mixture of Chl species. Fourth derivative and difference spectra showed the presence of minor bands at 692, 726 and 748 nm. The state of compression determines the quantity and type of spectral species formed. A Chl monolayer on an acid subphase results in the formation of a long wavelength absorbing species (705 nm) similar to that of pheophytin. The half-band width, optical density/monolayer, and extinction coefficients of Chl monolayers are given. It is concluded that in the monolayer the formation of various aggregated species of Chl can be induced.     

The Solutions of a Model Nonlinear Singular Perturbation Problem Having A Continuous Locus of Singular Points

Consider the boundary value problem εy″ =(y² ? t²)y′, ?1 ?t?0, y(?1) = A, y(0) = B. We discuss the multiplicity of solutions and their limiting behavior as ε→+0+ for certain choices of A and B. In particular, when A = 1, B = 0, a bifurcation analysis gives a detailed and fairly complete analysis. The interest here arises from the complexity of the set of "turning points."     

Determination of mycotoxins in human foods

This tutorial review deals with the analytical methods available for the determination of mycotoxins in food commodities. As the secondary metabolites of a range of fungal species, mycotoxins possess diverse chemical structures, presenting analytical chemists with a unique set of challenges in the microg kg(-1) (ppb) range. A number of analytical methods have been applied to mycotoxin analysis. These include widely applicable HPLC methods with UV or fluorimetric detection, which are extensively used both in research and for legal enforcement of food safety legislation and for regulations in international agricultural trade. Other chromatographic methods, such as TLC and GC, are also employed for the determination of mycotoxins, whereas recent advances in analytical instrumentation have highlighted the potential of LC-MS methods, especially for multi-toxin determination and for confirmation purposes. Conventional chromatographic methods are generally time consuming and capital intensive, and hence a range of methods, mostly based on immunological principles, have been developed and commercialised for rapid analysis. These methods include, among others, enzyme-linked immunosorbent analysis (ELISA), direct fluorimetry, fluorescence polarization, and various biosensors and strip methods.     

Rates of Convergence and Approximations to the Distribution of the Maximum of Chain-Dependent Sequences

In this paper we propose two methods for approximating the distribution of the maximum for a chain-dependent process. For these methods we investigate the rate of convergence of the approximations. A simulation study was performed that demonstrated excellent performance of the approximations even for small sample sizes. In addition, an application was made to river flow data collected on the North Pacolet River in South Carolina.     

Self-assembling poly( -lysine)/DNA complexes capable of integrin-mediated cellular uptake and gene expression

Integrin-mediated delivery of genes is evaluated using a synthetic vector formed by self-assembly of DNA with an oligolysine- peptide sequence containing RGD (referred to as K₁₆-RGD). The RGD peptide binds plasmid DNA effectively and inhibits ethidium bromide/DNA fluorescence at N-to-P ratios of less than 1.0. At N:P ratio 1.0, peptide/DNA complexes formed show a mixture of normal DNA migration and retention at the origin when analysed by agarose electrophoresis. At N:P ratio of 1.2, the complexes have a slight positive surface charge (5 mV) and in the absence of serum they show 10-fold increase uptake into 293 cells, compared with control poly( -lysine)/DNA vectors, together with a 100-fold increase in transfection. In the presence of serum, RGD-mediated uptake is decreased about 3-fold, but the targeted vectors achieve over 150 times greater transfection than poly( -lysine)/DNA controls. Transfection could be inhibited by addition of competing RGD, and to a lesser extent RGE, peptides. The targeted vector is believed to achieve cell uptake and transfection by binding av integrins in the cell surface, and the approach could be employed to promote internalisation of vectors following their binding to other, high affinity, receptors, in a system analogous to adenovirus entry.     

Long cycles in critical graphs

It is shown that any k‐critical graph with n vertices contains a cycle of length at least , improving a previous estimate of Kelly and Kelly obtained in 1954. © 2000 John Wiley & Sons, Inc. J Graph Theory 35: 193–196, 2000