Evaluation of the separation mechanism of electrokinetic chromatography with a microemulsion and cyclodextrins using NMR and molecular modeling

Electrokinetic chromatography (EKC) allows the separation of closely related substances by the detection of fine effects in analyte-separation system interactions. With the goal of understanding the fine effects involved in separation using a dual cyclodextrin-microemulsion EKC system, an integrated study of NMR and molecular modeling was carried out. The above dual cyclodextrin-microemulsion system was previously used in the separation of clemastine and its related substances and was prepared by the addition of methyl-β-cyclodextrin (MβCD) and heptakis(2,6-di-O-methyl)-β-cyclodextrin (DMβCD) to an oil-in-water microemulsion. The use of DMβCD was shown to be essential in the separation of clemastine from one of its related substance (I(B) ). A molecular modeling study allowed the different affinities of clemastine and I(B) for the two cyclodextrins to be explained. Furthermore, rotating-frame Overhauser effect spectroscopy NMR experiments clearly indicated that besides the primary pseudostationary phase, namely the ionic microemulsion, cyclodextrins acted as a secondary pseudostationary phase. In addition, it was shown that inclusion complexation of sodium dodecyl sulfate (SDS) monomers into the cyclodextrins cavity occurs; differently, the oil (n-heptane) used in the preparation of microemulsion system resulted to be not included into the macrocycle cavity. These experimental results were supported by molecular modeling, which highlighted the preferential inclusion of SDS into DMβCD. On the basis of these results, it was confirmed that, besides its primary role as the ionic carrier in EKC, SDS is involved in inclusion equilibria toward CDs, which can be effective in increasing the system selectivity.     

Decomposition of molybdate–hexamethylenetetramine complex: One single source route for different catalytic materials

Decomposition of ammonium heptamolybdate–hexamethylentetramine (HMTA) complex (HMTA)₂(NH₄)₄Mo₇O₂₄·2H₂O was studied as a function of treatment conditions in the range 300–1173 K. The evolution of solid products during decomposition was studied by thermal analysis and in situ EXAFS. Depending on the nature of the gas used for treatment, single phases of highly dispersed nitrides Mo₂N, carbide Mo₂C, or oxide MoO₂ can be obtained. The nature of the products obtained was explained by qualitative thermodynamical considerations. Morphology of the solids considerably depends on such preparation parameters as temperature and mass velocity of the gas flow. For the nitride-based materials, catalytic activity was evaluated in the model thiophene HDS reaction. It was demonstrated that NH₃-treated samples showed better catalytic activity than N₂-treated ones due to cleaner surface and better morphology. Transmission microscopy, XRD and XPS studies showed that MoS₂ is formed on the surface during HDS reaction or sulfidation with H₂S. Optimized nitride-derived catalysts showed mass activity several times higher than unsupported MoS₂ or MoS₂/Al₂O₃ reference catalyst.     

Post-translational modification in the gas phase: mechanism of cysteine S-nitrosylation via ion-molecule reactions

The gas-phase mechanism of S-nitrosylation of thiols was studied in a quadrupole ion trap mass spectrometer. This was done via ion-molecule reactions of protonated cysteine and many of its derivatives and other thiol ions with neutral tert-butyl nitrite or nitrous acid. Our results showed that the presence of the carboxylic acid functional group, -COOH, in the vicinity of the thiol group is essential for the gas-phase nitrosylation of thiols. When the carboxyl proton is replaced by a methyl group (cysteine methyl ester) no nitrosylation was observed. Other thiols lacking a carboxylic acid functional group displayed no S-nitrosylation, strongly suggesting that the carboxyl hydrogen plays a key role in the nitrosylation process. These results are in excellent agreement with a solution-phase mechanism proposed by Stamler et al. (J. S. Stamler, E. J. Toone, S. A. Lipton, N. J. Sucher. Neuron 1997, 18, 691-696) who suggested a catalytic role for the carboxylic acid group adjacent to cysteine residues and with later additions by Ascenzi et al. (P. Ascenzi, M. Colasanti, T. Persichini, M. Muolo, F. Polticelli, G. Venturini, D. Bordo, M. Bolognesi. Biol. Chem. 2000, 381, 623-627) who postulated that the presence of the carboxyl in the cysteine microenvironment in proteins is crucial for S-nitrosylation. A concerted mechanism for the gas-phase S-nitrosylation was proposed based on our results and was further studied using theoretical calculations. Our calculations showed that this proposed pathway is exothermic by 44.0 kJ mol(-1). This is one of the few recent examples when a gas-phase mechanism matches one in solution.     

Quantification of (−)-Epicatechin and Procyanidin B2 in Chocolates

JPC – Journal of Planar Chromatography – Modern TLC - Validation of a method for the quantification of (−)-epicatechin and procyanidin B2 in chocolates is reported. After a simple...     

Evaluation of automated sample preparation, retention time locked gas chromatography-mass spectrometry and data analysis methods for the metabolomic study of Arabidopsis species

In this paper, automated sample preparation, retention time locked gas chromatography-mass spectrometry (GC-MS) and data analysis methods for the metabolomics study were evaluated. A miniaturized and automated derivatisation method using sequential oximation and silylation was applied to a polar extract of 4 types (2 types×2 ages) of Arabidopsis thaliana, a popular model organism often used in plant sciences and genetics. Automation of the derivatisation process offers excellent repeatability, and the time between sample preparation and analysis was short and constant, reducing artifact formation. Retention time locked (RTL) gas chromatography-mass spectrometry was used, resulting in reproducible retention times and GC-MS profiles. Two approaches were used for data analysis. XCMS followed by principal component analysis (approach 1) and AMDIS deconvolution combined with a commercially available program (Mass Profiler Professional) followed by principal component analysis (approach 2) were compared. Several features that were up- or down-regulated in the different types were detected.     

Biocompatible quantum dots for biological applications

Semiconductor quantum dots are quickly becoming a critical diagnostic tool for discerning cellular function at the molecular level. Their high brightness, long-lasting, size-tunable, and narrow luminescence set them apart from conventional fluorescence dyes. Quantum dots are being developed for a variety of biologically oriented applications, including fluorescent assays for drug discovery, disease detection, single protein tracking, and intracellular reporting. This review introduces the science behind quantum dots and describes how they are made biologically compatible. Several applications are also included, illustrating strategies toward target specificity, and are followed by a discussion on the limitations of quantum dot approaches. The article is concluded with a look at the future direction of quantum dots.     

Divergent synthetic approach to 6'-modified α-GalCer analogues

A synthetic approach is presented for the synthesis of galacturonic acid and D-fucosyl modified KRN7000. The approach allows for late-stage functionalisation of both the sugar 6'-OH and the sphingosine amino groups, which enables convenient synthesis of promising 6'-modified KRN7000 analogues.     

Biomimetic cationic polyannulation reaction catalyzed by Bi(OTf)3: cyclization of 1,6-dienes, 1,6,10-trienes, and aryl polyenes

Nonactivated trienes and aryltrienes were cyclized into polycyclic compounds in good to excellent yields under bismuth triflate catalysis in a biomimetic fashion. The reaction showed broad applicability and allowed for the formation of functionalized bicyclic to tetracyclic structures from simple precursors in one pot. For some specific substrates, the cyclization was followed by a methyl shift as encountered in terpenoid biosynthesis.     

Phenyliodine(III) bis(trifluoroacetate) (PIFA)-promoted synthesis of Bodipy dimers displaying unusual redox properties

Phenyliodine(III) bis(trifluoroacetate) (PIFA) in conjunction with a Lewis acid promotes C-C coupling of Bodipy monomers leading to mixtures of various oligomers. When a single position is blocked with an iodo or phenyl group, formation of the dimer is favored. These dimers display two successive oxidation and two reduction waves separated on average by 260 and 130 mV, respectively, corresponding to each Bodipy subunit.