Application of MEKC and monolithic CEC for the analysis of bioactive naphthoquinones in Eleutherine americana

Two microscale separation techniques for the analysis of bioactive naphthoquinones in Eleutherine americana were developed and validated. By MEKC four compounds (eleuthoside B, isoeleutherin, eleutherol and eleutherinoside A) could be determined in plant extracts using an aqueous electrolyte solution composed of 25 mM sodium tetraborate, 50 mM sodium cholate and 20% THF. CEC on a polymeric methacrylate‐based monolith with strong cationic properties showed promising results, as it additionally enabled the separation of two enantiomers, eleutherin and isoeleutherin. The mobile phase for CEC experiments comprised 3 mM ammonium formate in a mixture of ACN and water. At an applied voltage of ?25 kV, all five markers were baseline separated in less than 12 min. Both methods were successfully validated for linearity (MEKC: R²≥0.999; CEC: R²≥0.997), sensitivity (MEKC: LOD=4–5 μg/mL; CEC: LOD=2–8 μg/mL), accuracy (MEKC: 96.5–102.7% recovery; CEC: 97.1–103.5% recovery) and precision (MEKC: σ_rel≤2.43%; CEC: σ_rel≤2.21%). The quantitative analysis of naphthoquinone derivatives in several E. americana samples showed that both methods are suitable for practical applications, because the results were well comparable to those obtained by established techniques such as HPLC.     

On a generic class of two-node queueing systems

This paper analyzes a generic class of two-node queueing systems. A first queue is fed by an on–off Markov fluid source; the input of a second queue is a function of the state of the Markov fluid source as well, but now also of the first queue being empty or not. This model covers the classical two-node tandem queue and the two-class priority queue as special cases. Relying predominantly on probabilistic argumentation, the steady-state buffer content of both queues is determined (in terms of its Laplace transform). Interpreting the buffer content of the second queue in terms of busy periods of the first queue, the (exact) tail asymptotics of the distribution of the second queue are found. Two regimes can be distinguished: a first in which the state of the first queue (that is, being empty or not) hardly plays a role, and a second in which it explicitly does. This dichotomy can be understood by using large-deviations heuristics. This work has been carried out partly in the Dutch BSIK/BRICKS project.     

Uncertainty in liquid chromatographic analysis of pharmaceutical product: influence of various uncertainty sources

An ISO GUM measurement uncertainty estimation procedure was developed for a liquid-chromatographic drug quality control method-assay of simvastatin in drug formulation. In quantification of uncertainty components several practical approaches for including difficult-to-estimate uncertainty sources (such as uncertainty due to peak integration, uncertainty due to nonlinearity of the calibration curve, etc.) have been presented. Detailed analysis of contributions of the various uncertainty sources was carried out. The results were calculated based on different definitions of the measurand and it was demonstrated that unequivocal definition of the measurand is essential in order to get rigorous uncertainty estimate. Two different calibration methods - single-point (1P) and five-point (5P) - were used and the obtained uncertainties and uncertainty budgets were compared. Results calculated using 1P and 5P calibrations agree very well. The uncertainty estimate for 1P is only slightly larger than with 5P calibration.     

Asymmetric allylic substitution catalyzed by C1-symmetrical complexes of molybdenum: structural requirements of the ligand and the stereochemical course of the reaction

Application of new chiral ligands (R)-(-)-12 a and (S)-(+)-12 c (VALDY), derived from amino acids, to the title reaction, involving cinnamyl (linear) and isocinnamyl (branched) type substrates (4 and 5 --> 6), led to excellent regio- and enantioselectivities (>30:1, < or =98 % ee), showing that ligands with a single chiral center are capable of high asymmetric induction. The structural requirements of the ligand and the mechanism are discussed. The application of single enantiomers of deuterium-labeled substrates (both linear 38 c and branched 37 c) and analysis of the products (41-43) by (2)H{(1)H} NMR spectroscopy in a chiral liquid crystal matrix allowed the stereochemical pathways of the reaction to be distinguished. With ligand (S)-(+)-12 c, the matched enantiomer of branched substrate was found to be (S)-5, which was converted into (R)-6 with very high regio- and stereoselectivity via a process that involves net retention of stereochemistry. The mismatched enantiomer of the branched substrate was found to be (R)-5, which was also converted into (R)-6, that is, with apparent net inversion, but at a lower rate and with lower overall enantioselectivity. This latter feature, which may be termed a "memory effect", reduced the global enantioselectivity in the reaction of the racemic substrate (+/-)-5. The stereochemical pathway of the mismatched manifold has been shown also to be one of net retention, the apparent inversion occurring through equilibration via an Mo-allyl intermediate prior to nucleophilic attack. Incomplete equilibration leads to the memory effect and thus to lower enantioselectivity. Analysis of the mismatched manifold over the course of the reaction revealed that the memory effect is progressively attenuated with the nascent global selectivity increasing substantially as the reaction proceeds. The origin of this effect is suggested to be the depletion of CO sources in the reaction mixture, which attenuates turnover rate and thus facilitates greater equilibrium. The linear substrate was also converted into the branched product with net syn stereochemistry, as shown by isotopic labeling. An analogous process operates in the generation of small quantities of linear product from branched substrate.     

Substituent effects on the basicity of 3,7-diazabicyclo[3.3.1]nonanes

Basicity constants for a series of 3,7-diazabicyclo[3.3.1]nonane derivatives in acetonitrile with a variation over 13 orders of magnitude have been determined using a spectrophotometric titration technique. An excellent correlation between basicity and calculated proton affinities obtained at PCM-B3LYP/6-31+G(d)//B3LYP/6-31G(d) level was found. The results are discussed in terms of substituent effects and compared to (15)N NMR chemical shifts.     

Low-Temperature High-Resolution Solid-State (cryoMAS) NMR of Han Purple BaCuSi2O6

We present low-temperature (low-T, sub-liquid-N₂) high-speed high-resolution ²⁹Si solid-state (cryoMAS) nuclear magnetic resonance studies on a model 2D-BEC quantum magnet BaCuSi₂O₆, known also as Han Purple. We observe broadened ²⁹Si lines below the well-established 100 K structural phase transition confirming the existence of inhomogeneities at low temperatures. Interestingly, the low-T spectra of \({\mathrm{BaCuSi}}_{2}{\mathrm{O}}_{6}\) closely resemble those of the novel compound Ba₂CoSi₂O₆Cl₂ taken at room temperature. This suggests that the Co compound features structural modulations or inhomogeneities already at room temperature. The low-T crystal structure and magnetism of \({\mathrm{BaCuSi}}_{2}{\mathrm{O}}_{6}\) are more complex than previously believed, and deserve further investigation.     

Simultaneous EEG-fMRI acquisition: how far is it from being a standardized technique?

Simultaneous EEG–fMRI is a powerful tool to study spontaneous and evoked brain activity because of the complementary advantages of the two techniques in terms of temporal and spatial resolution. In recent years, a significant number of scientific works have been published on this subject. However, many technical problems related to the intrinsic incompatibility of EEG and MRI methods are still not fully solved. Furthermore, simultaneous acquisition of EEG and event-related fMRI requires precise synchronization of all devices involved in the experimental setup. Thus, timing issue must be carefully considered in order to avoid significant methodological errors.

The aim of the present work is to highlight and discuss some of technical and methodological open issues associated with the combined use of EEG and fMRI. These issues are presented in the context of preliminary data regarding simultaneous acquisition of event-related evoked potentials and BOLD images during a visual odd-ball paradigm.     

Dimeric calixarenes: a new family of major-groove binders

A new class of potent DNA binding agents is presented. Dimeric calix[4]arenes with cationic groups at their upper rims and flexible alkyl bridges can be synthesized from triply acyl-protected calix[4]arene tetramines in relatively short synthetic sequences (3-5 steps). The compounds attach themselves to double-stranded nucleic acids in a noncovalent fashion, with micro- to nanomolar affinities. Guanidinium headgroups with their extended hydrogen-bonding "fingers" are more powerful than ammonium groups, and the benzylamine series is superior to the anilinium series (see below). The new ligands easily distinguish between RNA and various DNA types, and produce characteristic changes in UV/Vis, fluorescence, CD, as well as NMR spectra. Especially extended oligonucleotides of more than 100 base pairs are bound with affinities increasing from RNA (10?μM K(d))相似文献   

Excited state proton transfer in strongly enhanced GFP (sGFP2)

Proton transfer is an elementary process in biology. Green fluorescent protein (GFP) has served as an important model system to elucidate the mechanistic details of this reaction, because in GFP proton transfer can be induced by light absorption. We have used pump-dump-probe spectroscopy to study how proton transfer through the 'proton-wire' around the chromophore is affected by a combination of mutations in a modern GFP variety (sGFP2). The results indicate that in H(2)O, after absorption of a photon, a proton is transferred (A* → I*) in 5 ps, and back-transferred from a ground state intermediate (I → A) in 0.3 ns, similar to time constants found with GFPuv, although sGFP2 shows less heterogeneous proton transfer. This suggests that the mutations left the proton-transfer largely unchanged, indicating the robustness of the proton-wire. We used pump-dump-probe spectroscopy in combination with target analysis to probe suitability of the sGFP2 fluorophore for super-resolution microscopy.