A novel electrochemical approach using two different electrode materials, platinum and boron-doped diamond (BDD), was employed to study the oxidative stability of the drug abacavir. Abacavir samples ...were subjected to oxidation and subsequently analysed using chromatography with mass detection. The type and amount of degradation products were evaluated, and results were compared with traditional chemical oxidation using 3% hydrogen peroxide. The effect of pH on the rate of degradation and the formation of degradation products were also investigated. In general, both approaches led to the same two degradation products, identified using mass spectrometry, and characterised by 319.20 and m/z 247.19. Similar results were obtained on a large-surface platinum electrode at a potential of +1.15 V and a BDD disc electrode at +4.0 V. Degradation of 20% of abacavir, the rate required for pharmaceutical stability studies, took only a few minutes compared to hours required for oxidation with hydrogen peroxide. Measurements further showed that electrochemical oxidation in ammonium acetate on both types of electrodes is strongly pHdependent. The fastest oxidation was achieved at pH 9. The pH also affects the composition of the products, which are formed in different proportions depending on the pH of the electrolyte.
•Polar organic solvents yield fast and efficient enantioseparation of liquid crystals.•Chlorinated amylose-based chiral column brings the highest enantioselectivity values.•Methanol-rich mobile ...phases yield the best results for perfluorinated liquid crystals.•Lateral fluorination reduces enantioresolution regardless of the used mobile phase.
Three sets of fluorinated chiral liquid crystals were used to explore the polar organic solvent chromatography mode for their enantioseparation. The materials include a set of newly synthesized compounds with chiral center derived from 2-hexanol and two sets of compounds with chiral center derived from 2-/3-octanol. Baseline enantioseparation of all materials was achieved using binary mobile phases without additives. For some of the compounds exceedingly high values of enantioresolution (> 20) and enantioselectivity (> 4) were found. The chromatographic behavior of the sample set was studied on three different polysaccharide-based chiral columns – Chiralpak IA-U, IG-U and IB-U. Comparison of results from Chiralpak IA-U and IB-U shows the effect of amylose vs. cellulose polysaccharide backbone while comparison of Chiralpak IA-U and IG-U reveals the effect of 3,5-dimethylphenylcarbamate vs. 3‑chloro-5-methylphenylcarbamate substituent. The mobile phases tested included whole range of acetonitrile/methanol mixtures to demonstrate that acetonitrile-rich and alcohol-rich mobile phases offer different enantiorecognition mechanisms and can provide complementarity to some extent. The effect of temperature on enantioseparation was investigated on Chiralpak IA-U by constructing van't Hoff plots for selected liquid crystals in pure acetonitrile and pure methanol as mobile phases.
•Enantioselective chromatographic methods for chiral liquid crystals are reviewed.•Chiral liquid crystalline material can work as a chiral selector in separations.•Liquid crystals are used for ...fabrication of enantioselective materials and surfaces.•Lyotropic mesogens are indispensable media for NMR enantiomer differentiating.
Liquid crystals can partake in enantioseparations in several different ways. Enantiomeric purity of chiral liquid crystals as analytes is often determined using enantioselective liquid or supercritical fluid chromatography. At the same time, chiral liquid crystalline materials can be applied as a chiral selector or they can function as an auxiliary material for a different chiral selector enabling enantioseparation of various solutes in miscellaneous separation techniques. They can also be used in fabrication of matrices or surfaces sensitive towards enantiomers or employed as chiral solvents for visualizing enantiomers in NMR. In this review, all of these aspects of relationship between liquid crystals and enantiodiscrimination are discussed in an effort to encompass their versatility and with emphasis on enantioseparations.
A comprehensive study into the effects of mobile phase composition and column temperature on enantiomer elution order was conducted with a set of chiral rod‐like liquid crystalline materials. The ...analytes were structurally similar and comprised variances such as length of terminal alkyl chain, presence of chlorine, number of phenyl rings, and type of chiral center. Experiments were carried out in polar organic and reversed‐phase modes using amylose tris(3‐chloro‐5‐methylphenylcarbamate) immobilized on silica gel as the chiral stationary phase. For all liquid crystals, reversal of elution order of enantiomers was observed based on type of used cosolvent and/or its content in the mobile phase; for some of the liquid crystals a temperature‐induced reversal was also observed. Both linear and nonlinear dependencies of natural logarithm of enantioselectivity on temperature were found. Tested mobile phases comprised pure organic solvents and binary and tertiary mixtures of acetonitrile with organic solvents and/or water. Effect of acidic/basic mobile phase additives was also tested. Effect of structure of chiral selector is briefly discussed.
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•First UPSFC and UHPLC enantioseparation of lactic acid based liquid crystals.•Reversal of enantiomer elution order observed in both SFC and non-aqueous RPLC mode.•Complete ...enantioseparation of liquid crystals was achieved in under 6 min.
Fast and efficient ultra-performance chromatographic methods were developed for enantioseparation of new liquid crystals based on lactic acid. Ultra-performance supercritical fluid chromatography and non-aqueous reversed-phase liquid chromatography were applied for the first time for enantioseparation of this type of analytes. Both techniques proved to be suitable and baseline enantioseparation of all compounds was achieved with tris(3,5-dimethylphenylcarbamate) derivative of amylose as chiral selector. The compounds differentiate in number of benzene rings, length of alkyl side chains and presence of chlorine as lateral substituent and the effect of analyte structure on the enantioseparation was assessed. Significant effect of type of mobile phase modifier was observed in both systems including switching the enantiomer elution order based on both the type of the modifier (both techniques) and its portion in mobile phase (non-aqueous reversed-phase mode only).
A fast and simple supercritical fluid chromatography method for the enantioseparation of twenty newly synthesized orthoconic antiferroelectric liquid crystals is reported for the first time. The ...effects of alkoxy spacer length and fluorine atom presence and position in the phenyl ring on chromatographic behavior were investigated. Baseline enantioseparation of all compounds was achieved using simple mobile phases consisting of carbon dioxide and alcohol as cosolvent on (3,5-dimethylphenylcarbamate) derivative of amylose as chiral stationary phase. The analysis times ranged from 2 to 4 and from 4 to 7 min for most samples when using methanol and propane-2-ol, respectively. The significant effect of cosolvent type on the enantioseparation of these compounds was assessed and partial complementarity of methanol and propane-2-ol was observed.
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•SFC enantioseparation of liquid crystals is reported for the first time.•Complementarity of methanol and propane-2-ol as cosolvents was shown.•The effect of alkoxy spacer length and fluorine atom presence and position on enantioseparation was substantial.
Domains rich in cationic amino acids are ubiquitous in peptides with the ability to cross cell membranes, which is likely related to the binding of such polypeptides to anionic groups on the membrane ...surface. To shed more light on these interactions, we investigated specific interactions between basic amino acids and oligopeptides thereof and anions by means of electrophoretic experiments and molecular dynamics simulations. To this end, we measured the electrophoretic mobilities of arginine, lysine, tetraarginine, and tetralysine in sodium chloride and sodium sulfate electrolytes as a function of ionic strength. The mobility was found to be consistently lower in sodium sulfate than in sodium chloride at the same ionic strength. The decrease in mobility in sodium sulfate was greater for tetraarginine than for tetralysine and was larger for tetrapeptides compared to the corresponding free amino acids. On the basis of molecular dynamics simulations and Bjerrum theory, we rationalize these results in terms of enhanced association between the amino acid side chains and sulfate. Simulations also predict a greater affinity of sulfate to the guanidinium side chain groups of arginine than to the ammonium groups of lysine, as the planar guanidinium geometry allows simultaneous strong hydrogen bonding to two sulfate oxygens. We show that the sulfate binding to arginine, but not to lysine, is cooperative. These results are consistent with the greater decrease in the mobility of arginine compared to that of lysine upon addition of sulfate salt. The nonspecific mobility retardation by sulfate is ascribed to its electrostatic interaction with the cationic amino acid side chain groups.
Using capillary electrophoresis and molecular dynamics simulations, we directly demonstrate that guanidinium cations exhibit an affinity for the positively charged side chains of arginine-containing ...peptides in aqueous solutions, while no such effect is observed for sodium cations and/or lysine. Such a counterintuitive pairing between these two positively charged moieties has implications for destabilization and aggregation of arginine-rich proteins in guanidinium salt solutions.
Enantiomeric excess (EE) highly affects the mesomorphic properties of mesogens containing centre of chirality. Thus, there is need of methods for evaluation of the EE of newly synthesised compounds ...as it should be considered when discussing the mesomorphic properties. We report synthesis and characterisation of ten new racemic mixtures based on (R,S)-3-octanol, which were used to develop an enantioseparation method that can be used for the assessment of the EE of chiral liquid crystals and their mixtures. The compounds vary in the length of the oligomethylene spacer and lateral fluorine substitution of the aromatic core. The mesomorphic properties of novel materials were measured. Supercritical fluid chromatography with polysaccharide-based chiral stationary phases was used to develop reliable method for enantioseparation of the materials. A complete separation of (S) and (R) enantiomers with resolution values higher than 2.0 was obtained for all materials using simple mobile phase. The enantiomer elution order was assessed using pure (S) enantiomers of all materials. For all materials (R) enantiomer elutes first which is advantageous for use as optical purity control of newly synthesised (S) enantiomers.
Studie z roku 1998 zabývající se negativními zkušenostmi v dětství odhalila nejen vysokou prevalenci v běžné populaci, nýbrž i jejich rozsáhlý a zásadní dopad na duševní a také fyzické zdraví ...člověka v průběhu celého života. V reakci na alarmující výsledky studie lze sledovat celosvětový trend zvyšování podpory duševního zdraví všech žáků ve školách s ohledem na specifické možnosti podpory žáků, kteří v dětství zažili či zažívají negativní zkušenosti ovlivňující jejich duševní zdraví, chování a vzdělávání. Následující text představuje základní principy trauma respektujícího přístupu ve vzdělávání a předkládá možnost jeho systémové implementace zavedením celoškolního rámce pozitivní podpory chování PBIS. Cílem textu je diskutovat vhodnost zavedení víceúrovňové podpory dětí s negativními zkušenostmi v dětství projevujícími se problémy v chování prostřednictvím celoškolní tříúrovňové pozitivní podpory chování PBIS. Příspěvek je dělen do šesti částí. Po krátkém úvodu do problematiky je pozornost věnována výskytu a dopadům negativních zkušeností v dětství na život jedince, současným možnostem podpory dětí s negativními zkušenostmi v českém školství, základním principům trauma respektujícího přístupu, systémovému zavedení trauma respektujícího přístupu prostřednictvím rámce PBIS a možnostem implementace PBIS v českých školách. Rozbor odborné literatury odkrývá společné prvky PBIS s takzvaným trauma respektujícím přístupem, jež podporuje duševní zdraví všech žáků a současně umožňuje školám vhodně podporovat také žáky, kteří prožili nebo prožívají negativní zkušenosti. Implementační rámec PBIS je tedy vhodným systémem podpory duševního zdraví dětí a mladých lidí, který je možné adaptovat na české vzdělávání a zavádět v běžných školách.
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