Gray code for derangements

We give a Gray code and constant average time generating algorithm for derangements, i.e., permutations with no fixed points. In our Gray code, each derangement is transformed into its successor either via one or two transpositions or a rotation of three elements. We generalize these results to permutations with number of fixed points bounded between two constants.     

A study of the static yield stress in a binary Lennard-Jones glass

The stress-strain relations and the yield behavior of a model glass (a 80:20 binary Lennard-Jones mixture) is studied by means of molecular dynamics simulations. In a previous paper it was shown that, at temperatures below the glass transition temperature, Tg, the model exhibits shear banding under imposed shear. It was also suggested that this behavior is closely related to the existence of a (static) yield stress (under applied stress, the system does not flow until the stress sigma exceeds a threshold value sigmay). A thorough analysis of the static yield stress is presented via simulations under imposed stress. Furthermore, using steady shear simulations, the effect of physical aging, shear rate and temperature on the stress-strain relation is investigated. In particular, we find that the stress at the yield point (the "peak"-value of the stress-strain curve) exhibits a logarithmic dependence both on the imposed shear rate and on the "age" of the system in qualitative agreement with experiments on amorphous polymers, and on metallic glasses. In addition to the very observation of the yield stress which is an important feature seen in experiments on complex systems like pastes, dense colloidal suspensions and foams, further links between our model and soft glassy materials are found. An example is the existence of hysteresis loops in the system response to a varying imposed stress. Finally, we measure the static yield stress for our model and study its dependence on temperature. We find that for temperatures far below the mode coupling critical temperature of the model (Tc = 0.435 in Lennard-Jones units), sigmay decreases slowly upon heating followed by a stronger decrease as Tc is approached. We discuss the reliability of results on the static yield stress and give a criterion for its validity in terms of the time scales relevant to the problem.     

Novel RPLC stationary phases for lipophilicity measurement: solvatochromic analysis of retention mechanisms for neutral and basic compounds

An RPLC was developed to rapidly determine lipophilicity of neutral and basic compounds using three base deactivated RPLC stationary phases particularly designed for the analysis of basic compounds, namely, Supelcosil ABZ(+)Plus, Discovery RP Amide C16, and Zorbax Extend C18. The work consisted of three sets of experiments. In the first log kw values of neutral compounds were extrapolated using hydroorganic mobile phases at different compositions. Good correlation between log kw and log Poct indicated that the method was appropriate for these supports, without adding a silanol masking agent. In the second set of experiments, isocratic log k values of neutral and basic compounds were measured with three different mobile phases. The best estimation of lipophilicity was obtained for neutral and basic compounds when the secondary interactions were strongly reduced (i. e., when basic compounds were under their neutral form). In the third set of experiments, isocratic retention factors of basic compounds (in their neutral form) were measured with a high-pH mobile phase, on a chemically stable support (Zorbax Extend C18). Under these chromatographic conditions, correlation between the isocratic retention factors and log Poct (log D10.5) for basic compounds was similar to that for neutral compounds.     

Flow-injection amperometric determination of pesticides on the basis of their inhibition of immobilized acetylcholinesterases of different origin

Determination of the organophosphorus pesticides paraoxon, chlorpyrifos oxon, and malaoxon has been performed by a method based on inhibition of acetylcholinesterase (AChE) and amperometric detection in a flow-injection system with enzymes obtained from the electric eel (eeAChE) and Drosophila melanogaster (dmAChE) and immobilized on the surface of platinum electrode within a layer of poly(vinyl alcohol) bearing styrylpyridinium groups. dmAChE is more sensitive than eeAChE to inhibition by chlorpyrifos oxon and paraoxon. The sensitivity difference was largest for chlorpyrifos oxon (detection limit approx. 17 times lower), and practically none for malaoxon. Determination of the analytes in spiked river water samples by use of the dmAChE biosensor resulted in recoveries from 50 to 90 % for chlorpyrifos oxon at levels of 20 to 40 nmol L(-1), 50 to 100 % for paraoxon at 0.6 to 0.8 micro mol L(-1), and 140 to 190 % for malaoxon at 0.6 to 1.2 micro mol L(-1).     

Epitope extraction technique using a proteolytic magnetic reactor combined with Fourier-transform ion cyclotron resonance mass spectrometry as a tool for the screening of potential vaccine lead peptides

Epitope extraction technique is based on the specific digestion of a target protein followed by immunoaffinity isolation of a specific recognition peptide. This technique, in combination with mass spectrometry, has been efficiently used for epitope identification. The major goal of this work was to utilize newly developed enzyme and immunoaffinity magnetic reactors for the epitope extraction procedure and confirm the efficiency of this improved system for epitope screening of proteins. Alginic acid-coated magnetite microparticles with immobilized TPCK-trypsin provided high working efficiency with low non-specific adsorption, digestion time in minutes and low frequency of missed cleavages. The sensitivity and specificity of tryptic fragmentation of the beta-amyloid-peptide Abeta (1-40) as a model polypeptide was confirmed by Fourier-transform ion cyclotron resonance mass spectrometry analysis. The Sepharose reactor or immunoaffinity magnetic reactors, both with anti-amyloid-beta monoclonal antibodies, were used for specific isolation and identification of target peptides. In this way, the epitope extraction technique combined with mass spectrometric analysis is shown to be an excellent base for molecular screening of potential vaccine lead proteins.     

Comparison between phenylboronic acid and iron loaded silicas to selectively preconcentrate catecholamines on-line coupled with HPLC

Summary The use of the on-line preconcentration of catecholamines on small precolumns packed with iron loaded and phenylboronic acid silicas was investigated. For both preconcentration materials, parameters which can influence the retention of catecholamines on the support such as pH, ionic strength, polarity and flow rate of the sample solution are described to predict the mechanism of retention on these silicas. On-line coupling with an ion-pair chromatography column is presented. Combined with amperometric detection, these methods have been applied to the quantitative determination of catecholamines in urine samples, using 1,2-dihydroxybenzylamine as an internal standard.     

Optimization of derivatization parameters with 9-fluorenylmethyl chloroformate for amphetamine and catecholamine separation by supercritical fluid chromatography

Summary The separation of amino compounds by supercritical fluid chromatography (SFC) is a difficult problem to solve, owing to the apolar nature of CO₂. The derivatization of amino functions with the 9-fluorenylmethyl chloroformate (FMOC-Cl) allows to obtain apolar UV-absorbing compounds easily eluted with a supercritical mobile phase. Optimization of derivatization parameters allows us to analyze quantitatively amphetamines and catecholamines. These compounds can be separated in less than 5 min with a small addition of methanol as polar modifier. The total procedure takes no more than 15 min and can be automatized to gain time. As presented in this study, this method can be employed to physiological fluids as urine.     

Development of a bioreactor based on trypsin immobilized on monolithic support for the on-line digestion and identification of proteins

The preparation and characterization of a new trypsin-based bioreactor is here described for on-line protein digestion and peptide analysis. Trypsin was immobilized on an epoxy-modified silica monolithic support with a single reaction step and the amount of immobilized enzyme was found to be 66.07 mg (+/-11.75 S.D.)/column (n = 6). The bioreactor was coupled through a switching valve to an analytical column for the on-line digestion, peptide separation and identification of test proteins by ESI-MS-MS. The influence of various parameters (flow rate, temperature, buffer pH and molarity, etc.) on enzymatic activity was investigated by an experimental design and the mostly significant factor was found to be the flow rate. The efficacy of the reported on-line bioreactor for tryptic mapping is reported for somatostatin and myoglobin, selected as model compounds. Tryptic peptide maps obtained by on-line digestion of myoglobin were compared to those obtained by traditional off-line digestion. Sequence coverage obtained with the on-line protocol (21 peptides, 75.16% coverage of myoglobin sequence) was found to be comparable to the one obtained with the off-line protocol (18 peptides, 76.47% coverage). Sensitivity for myoglobin digestion and identification was 0.1 mg/ml. The reproducibily of the peptide maps in terms of retention time was from 1.53 to 4.31%, R.S.D.     

Capillary electrophoresis-diode array detection--electrospray mass spectrometry for the analysis of selected tropane alkaloids in plant extracts

Capillary zone electrophoresis, coupled to UV and interfaced with electrospray ionization mass spectrometry (ESI-MS), is described for the simultaneous analysis of hyoscyamine and scopolamine. On-line UV detection occurred at 22 cm from the inlet of the capillary and ESI-MS monitoring was performed along the entire length of the capillary (85 cm). An alkaline solution of 40 mM ammonium acetate at pH 8.5 was suitable for the analysis of the alkaloids under consideration. Under the optimized conditions, including CE and ESI-MS parameters, the two alkaloids were resolved within a short time and with very high sensitivity. The differentiation of hyoscyamine and its positional isomer littorine, commonly encountered in plant material, is also presented using up-front collision-induced dissociation. Finally, the developed method was applied to the analysis of these alkaloids in Belladonna leaf extract and in Datura candida x D. aurea hairy root extract.     

Solid-state NMR study of intercalated species in poly(epsilon-caprolactone)/clay nanocomposites

The structure and dynamics of surfactant and polymer chains in intercalated poly(epsilon-caprolactone)/clay nanocomposites are characterized by (31)P magic-angle spinning (MAS) and (13)C cross-polarization MAS NMR techniques. To obtain hybrid materials with the low polymer content required for this study, in situ intercalative polymerization was performed by adapting a published procedure. After nanocomposite formation, the chain motion of the surfactant is enhanced in the saponite-based materials but reduced in the Laponite ones. Compared to the starting clay, the trans conformer population of the surfactant hydrocarbon chain in the nanocomposite decreases for the saponite systems. Mobility of the polymer chain is higher in the nanocomposites than in the bulk phase. The charge of the modified saponite does not significantly influence chain mobility in the nanocomposites.