Computer Study of the Association Behavior of Gradient Copolymers: Analysis of Simulation Results Based on a New Algorithm for Recognition and Classification of Aggregates

We present a computer study of the association behavior of copolymer chains with a gradient part and soluble tail of variable length. As a simulation method we use dynamic Monte Carlo simulation on a simple cubic lattice with pair interaction parameters. The solvent quality and selectivity is modeled by the variation of pair interaction parameters between nearest neighbors on the lattice. The role of the length of soluble part in the self‐assembly and its effect on the structure of aggregates was the main goal of this work. The size and structure of aggregates were analyzed using an improved topological classification method which has been developed and tested in the present study. The structure and association numbers of aggregates were compared with those of linear diblock copolymers.

     

Critical evaluation of microextraction pretreatment techniques – Part 1: Single drop and sorbent‐based techniques

Sample pretreatment techniques or preconcentration constitute a very important step before the analysis of environmental, clinical, pharmaceutical, and other complex samples. Thanks to extraction techniques it is possible to achieve higher method sensitivities and selectivities. Miniaturization microextraction methods make them more environmentally friendly and only small amounts of samples are required. In the past 30 years, a number of microextraction methods have been developed and used and are documented in thousands of articles. Many reviews have been written focusing on their use in specified professional fields or on the latest trends. Unfortunately, no uniform nomenclature has been introduced for these methods. Therefore, this review attempts to classify all the essential microextraction techniques and describes their advantages, disadvantages, and the latest innovations. The methods are divided into two main groups: single drop and sorbent‐based techniques according to the type of extraction phase.     

Vapour permeation and sorption in fluoropolymer gel membrane based on ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulphonyl)imide

The emissions of hydrocarbons from fossil fuels into atmosphere entail both an economic loss and an environmental pollution. Membrane separations can be used for vapour recovery and/or vapour removal from the permanent gas stream, given that the appropriate membrane is identified. A neat poly(vinylidene fluoride-co-hexafluoropropylene) membrane is impermeable to both the representatives of aliphatic hydrocarbons and branched hydrocarbons, namely hexane and isooctane, whereas the permeation flux is enhanced by the presence of 80 mass % of the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulphonyl)imide in the membrane, as detailed in this work. The permeabilities of hydrocarbon vapours were determined from the binary mixture containing hydrocarbon and nitrogen to simulate the real input of an air stream containing a condensable hydrocarbon. The diffusion coefficient determined from sorption measurements was higher for hexane, as would be expected for a smaller molecule, whereas both the sorption isotherms and permeabilities of the hydrocarbons studied were found to be almost identical. It is possible that the sorption effect predominates in the transport mechanism for VOCs/N₂ separations.     

Controlled proteolysis of normal and pathological prion protein in a microfluidic chip

A microreactor for proteinase K (PK)-mediated protein digestion was developed as a step towards the elaboration of a fully integrated microdevice for the detection of pathological prion protein (PrP). PK-grafted magnetic beads were immobilized inside a polydimethylsiloxane (PDMS) microchannel using a longitudinal magnetic field parallel to the flow direction and a magnetic field gradient, thereby forming a matrix for enzymatic digestion. This self-organization provided uniform pore sizes, a low flow resistance and a strong reaction efficiency due to a very thin diffusion layer. The microreactor's performance was first evaluated using a model substrate, succinyl-ala-ala-ala-paranitroanilide (SAAAP). Reaction kinetics were typically accelerated a hundred-fold as compared to conventional batch reactions. Reproducibility was around 98% for on-chip experiments. This microsystem was then applied to the digestion of prion protein from brain tissues. Controlled proteolysis could be obtained by varying the on-chip flow rate, while a complete proteolysis of normal protein was achieved in only three minutes. Extracts from normal and pathological brain homogenates were finally compared and strong discrimination between normal and pathological samples was demonstrated.     

Dependence of the L-alanyl-L-alanine conformation on molecular charge determined from ab initio computations and NMR spectra

The l-alanyl-l-alanine (AA) molecule behaves differently in acidic, neutral, and basic environments. Because of its molecular flexibility and strong interaction with the aqueous environment, its behavior has to be deduced from the NMR spectra indirectly, using statistical methods and comparison with ab initio predictions of geometric and spectral parameters. In this study, chemical shifts and indirect spin-spin coupling constants of the AA cation, anion, and zwitterion were measured and compared to values obtained by density functional computations for various conformers of the dipeptide. The accuracy and sensitivity of the quantum methods to the molecular charge was also tested on the (mono)-alanine molecule. Probable AA conformers could be identified at two-dimensional potential energy surfaces and verified by the comparison of the computed parameters with measured NMR data. The results indicate that, whereas the main-chain peptide conformations of the cationic (AA+) and zwitterionic (AAZW) forms are similar, the anion (AA-) adopts also another, approximately equally populated conformer in the aqueous solution. Additionally, the NH2 group can rotate in the two main chain conformations of the anionic form AA-. According to a vibrational quantum analysis of the two-dimensional energy surfaces, higher-energy conformers might exist for all three charged AA forms but cannot be detected directly by NMR spectroscopy because of their small populations and short lifetimes. In accord with previous studies, the NMR parameters, particularly the indirect nuclear spin-spin coupling constants, often provided an excellent probe of a local conformation. Generalization to peptides and proteins, however, has to take into account the environment, molecular charge, and flexibility of the peptide chain.     

Pore structure of pyrolyzed scrap tires

Internal structure of carbon black produced by pyrolysis (CBp) of rubber samples from the top and bottom parts of sidewall and tread of a passenger car tire was investigated in nitrogen flow at different temperatures. The pore structure (specific surface area, pore size distribution, and porosity) of CBp and commercial CB, was compared. The development of pore structure and the increase of the specific surface area were most intensive during the thermal decomposition at temperatures ranging from 300°C to 500°C. This is caused by the intensive release of volatiles during the pyrolysis. After the pyrolysis was finished, at temperatures above 500°C, further decomposition of solid matter was associated with a slight increase of the specific surface area. Presented at the 34th International Conference of the Slovak Society of Chemical Engineering, Tatranské Matliare, 21–25 May 2007.     

Use of 6-<Emphasis Type="Italic">O</Emphasis>-mono-substituted derivatives of β-cyclodextrin-bearing substituent with two permanent positive charges in capillary electrophoresis

This study details the use of two permanently positively charged mono-substituted β-cyclodextrin derivatives, 6^I -deoxy-6^I-(N,N N′,N′,N^t’-pentamethyl-ethylene-1,2-diammonio)-cyclomaltoheptose dichloride (PEMEDA-β-CD) and the newly synthesised 6^I-deoxy-6^I-(N,N,N′,N′,N′-pentamethyl-propylene-1,3-diammonio)-cyclomaltoheptose (PEMPDA-β-CD) as chiral selectors in capillary electrophoresis. Cyclodextrin (CD) derivatives were tested as additives in various buffers at various pH values with the optional addition of an organic modifier. Fourteen anionogenic analytes were tested, including native amino acids, N-blocked amino acids and profens, which were detected using a UV-VIS detector at optimal wavelengths of 214 nm, 254 nm or 280 nm. A borate buffer (15 mmol L^?1) at pH 9.5 without the addition of an organic modifier was chosen as a suitable background electrolyte. In addition, the effect of the concentration of the chiral selector on the separation and enantioseparation of selected analytes was monitored. The additions of cyclodextrin derivatives varied within the concentration range of 0.0–5.0 mmol L^?1. Both chiral selectors were suitable for the enantioseparation of N-Boc-d,l-tryptophan, which was already separated on the baseline at 0.5 mmol L^?1 concentration of the chiral selector.     

Separation and quantification of 9-(alkylthio)acridines by capillary micellar electrokinetic chromatography and capillary liquid chromatography

Various thioacridine derivatives are potential chemotherapeutics against various diseases which are intensively synthesized, characterized, and investigated by many research groups. Efficient, fast, and reliable separation and quantification methods for their analysis are still to be developed. MEKC and capillary LC (CLC) were applied for the separation and quantification of five highly hydrophobic, weakly basic, and structurally similar 9-(alkylthio)acridines. Since the common anionic and cationic surfactants failed to separate the strongly hydrophobic thioacridines by MEKC, sodium cholate was used in an alkaline BGE and successfully employed for their fast separation. In CLC, the weakly basic nature of the thioacridines necessitated use of LiChrosorb RP-select B sorbent as the stationary phase, which combined with a very simple mobile phase methanol/water yielded an efficient chromatographic separation system. Both, the MEKC and CLC optimized separation methods were then applied to quantify the thioacridines within a concentration range of 1.0 x 10(-5)-1.0 x 10(-3) mol/L and the obtained experimental results were critically compared. In practical terms, the MEKC analytical method can quantify the analytes much faster but with a lower reliability while the CLC method performs slower analysis with a higher repeatability of the experimental results.     

Equivalency for Disconjugate Operators

The solutions of the dfferential equation L_nx + a₀(t)x = 0, where L_n is a disconjugate operator and a₀ is of one sign, are studied according to their behavior as t → ∞. We prove equivalence theorems between solutions of this equation and its adjoint in terms of property A and property B. These theorems generalize the results known for n = 3 and for odd order binomial equations. Some applications are given too.     

Conformation of Chains in Cores of Block Copolymer Micelles with Solubilized Homopolymer: a Monte Carlo Study

A system of compatible self‐avoiding polymer chains solubilized in spherical cores of block copolymer micelles was studied by lattice Monte Carlo simulations. The core is modeled as a spherical cavity on a simple cubic lattice, filled in partially by tethered (core‐forming) chains and partially by free (solubilized) chains. Molecular parameters (e.g., the ratio of the contour length of the model chains to the core radius) correspond to those in real micellar systems. The density (the fraction of lattice sites occupied) is 0.6 which corresponds to swollen micellar cores in real micellar systems. Simulations yield a constant segment density profile in the core. Both the tethered and solubilized chains acquire an ellipsoidal shape. The ellipsoids equivalent to both types of chains are more spherical than those in a melt and strongly oriented. The chains in the core show a Gaussian‐like behavior. Minor deviations from Gaussian behavior for tethered chains are due to surface effects.