Comparison of quantitative methods for analysis of polyphasic pharmaceuticals

Adequate very sensitive quantification methods are needed for the development and are also now required for the monitoring of undesirable solid form(s) as routine tests. The pre-requisite for quantitation are selectivity, sensitivity and most important the purity of standards and their proper storage, what is a challenge for metastable forms. Several analytical techniques are available such as X-ray diffraction, spectroscopy, thermal analysis and microcalorimetry. The different steps of the validation of the analytical methods and problems to be solved are discussed. Examples illustrate the different techniques and compare their possible advantages and limits. The relative standard deviation of measurements should allow for checking the homogenization procedure of mixtures for calibration. The validation should be carried out following ICH guidelines for validation of analytical methods. Comparison of different techniques in adequate concentration range add confidence in the analytical results.     

Analysis of methoxypyrazines in wine using headspace solid phase microextraction with isotope dilution and comprehensive two-dimensional gas chromatography

This study reports an optimized headspace-solid phase microextraction (HS-SPME) method for the determination of methoxypyrazines in wine. Analysis was performed by using comprehensive two-dimensional gas chromatography with novel detection capabilities, including nitrogen phosphorus detection (GC x GC-NPD) and time-of-flight mass spectrometry (GC x GC-TOFMS). In the latter, stable isotope dilution was performed for the quantitation of 2-methoxy-3-(2-methylpropyl) pyrazine (IBMP), using labelled 2-(2H3)methoxy-3-(2-methylpropyl)pyrazine (d3-IBMP) as the internal standard, and resolution of the two analogues was facilitated using the deconvolution capabilities of the TOFMS. This research represents the first report of HS-SPME with isotope dilution and GC x GC-TOFMS (GC x GC-IDTOFMS). Analysis by GC x GC-NPD enabled detection limits of 0.5 ng/L for the quantitation of IBMP, which was superior to that obtained using GC x GC-IDTOFMS (1.95 ng/L). Nevertheless, both methods were adequately sensitive for real wine analysis, yielding highly comparable IBMP concentrations of 26.1 and 27.8 ng/L, respectively, from a Sauvignon blanc wine. The complexity of the real wine headspace was simplified as a result of selective detection using GC x GC-NPD and, in the case of GC x GC-IDTOFMS, the use of extracted ion chromatograms (EICs).     

Nanostructured nonionic thymidine nucleolipid self-assembly materials

Three nucleoside lipids have been synthesized: 3'-oleoylthymidine, 3',5'-dioleoylthymidine, and 3'-phytanoylthymidine. Differential scanning calorimetry and X-ray diffraction have been employed to characterize the physical properties of these neat lipids. Polarizing optical microscopy, small-angle X-ray scattering, and cryo-transmission electron microscopy techniques have been used to investigate the phase behavior in aqueous systems. Both oleoyl-based nucleoside lipids adopted a lamellar crystalline phase in the neat form at room temperature, and the phytanoyl derivative exhibited a fluid isotropic phase. Under excess water conditions, the presence of one branched (phytanoyl) or one unsaturated (oleoyl) chain promoted the formation of a liquid-crystalline lamellar phase at physiological temperatures. In contrast, the 3',5'-dioleoylthymidine derivative is nonswelling and does not exhibit lyotropic liquid-crystalline phase behavior. The nucleolipids' propensity for DNA-type binding and recognition has been evaluated by using a monolayer system to measure surface pressure-area isotherms in a Langmuir trough and indicates that the nucleoside base is available for nonspecific hydrogen bonding in the monolayer liquid expanded state for the single-chain nucleolipids but not for the dual-chain amphiphile.     

Selective enrichment and identification of azide-tagged cross-linked peptides using chemical ligation and mass spectrometry

Protein-protein interaction is one of the key regulatory mechanisms for controlling protein function in various cellular processes. Chemical cross-linking coupled with mass spectrometry has proven to be a powerful method not only for mapping protein-protein interactions of all natures, including weak and transient ones, but also for determining their interaction interfaces. One critical challenge remaining in this approach is how to effectively isolate and identify cross-linked products from a complex peptide mixture. In this work, we have developed a novel strategy using conjugation chemistry for selective enrichment of cross-linked products. An azide-tagged cross-linker along with two biotinylated conjugation reagents were designed and synthesized. Cross-linking of model peptides and cytochrome c as well as enrichment of the resulting cross-linked peptides has been assessed. Selective conjugation of azide-tagged cross-linked peptides has been demonstrated using two strategies: copper catalyzed cycloaddition and Staudinger ligation. While both methods are effective, Staudinger ligation is better suited for enriching the cross-linked peptides since there are fewer issues with sample handling. LC MSⁿ analysis coupled with database searching using the Protein Prospector software package allowed identification of 58 cytochrome c cross-linked peptides after enrichment and affinity purification. The new enrichment strategy developed in this work provides useful tools for facilitating identification of cross-linked peptides in a peptide mixture by MS, thus presenting a step forward in future studies of protein-protein interactions of protein complexes by cross-linking and mass spectrometry.     

Reactions of terminal polyynes with benzyl azide

Terminal di-, tri-, tetra-, and pentaynes substituted with a variety of functional groups react with benzyl azide in the presence of CuSO(4)·5H(2)O and ascorbic acid to give derivatives of 4-ethynyl-, 4-butadiynyl-, 4-hexatriynyl-, and 4-octatetraynyl-1,2,3-triazoles in moderate to good yields. These reactions appear to proceed regioselectively, and functionalization occurs exclusively at the terminal alkyne moiety. As well, no evidence of multiple azide additions to the polyyne framework is observed. X-ray crystallographic analysis of nine derivatives is used to document the regioselectivity of the reaction as well as outline structural characteristics of the 1,2,3-triazole products.     

Variance of the virtual displacement

We show that the affine structure of the 3-dimensional space is deeply enmeshed with the static laws expression. The relevance of tensorial rules for calculus in mechanics is thus enhanced. The virtual work principle is stated precisely but a little twist is given to the usual statements: the tensorial nature of the so-called virtual displacement vector is asserted to be covariant. To cite this article: C. Vallée et al., C. R. Mecanique 332 (2004).

Résumé

Nous montrons que la structure affine de l'espace tridimensionnel est fortement imbriquée avec l'expression des lois de la statique. La pertinence de l'application des règles du calcul tensoriel à la mécanique est ainsi renforcée. Le principe des travaux virtuels est établi précisément avec une seule entorse aux exposés classiques : la nature tensorielle de l'habituel vecteur déplacement virtuel est affirmée covariante. Pour citer cet article : C. Vallée et al., C. R. Mecanique 332 (2004).     

Effect of nonlinear strain paths on forming limits under isotropic and anisotropic hardening

The path-dependence of the conventional Forming Limit Diagram (FLD) is an important issue for its applications in industry. Great efforts have been made to understand the nature of the path-dependence with both experimental and theoretical approaches, many of them attempting to find a path-independent way for the application of forming limits. In this paper, we focus on the nonlinear strain path effect on forming limit predictions using both isotropic and anisotropic hardening models. The Forming Limit Diagram (FLD), Forming Limit Stress Diagram (FLSD) and Forming Limit Effective Strain Diagram (epFLD) of sheet metals subject to linear and nonlinear strain paths are analyzed and compared using the Marciniak–Kuczynski approach. An anisotropic hardening model based on Yoshida and Uemori development is adopted in this study, and it is coupled with the traditional Hill’48 yield surface. This model is capable of describing the complex Bauschinger phenomenon after the material undergoes the reverse loading process such as the early re-yielding, work-hardening stagnation and permanent softening. Two different scenarios for the change of strain paths are also investigated. In the first scenario, the sheet material is initially loaded with a fixed strain increment ratio, unloaded to the free stress state, and then reloaded with a different strain increment ratio until the forming limit is reached. In the second scenario, the material does not undergo elastic unloading. Instead, the strain path is abruptly changed to a different strain increment ratio and the material undergoes continuous loading until the forming limit is reached. It is found that the work-hardening behavior after the pre-straining and the loading scenario plays an important role in the path dependent behavior of forming limits. Detailed analysis reveals that the M–K approach may have contributed to the significance of path-dependence observed in this study, especially at high pre-strain levels.     

A Study of Student Engagement in Project‐Based Learning Across Multiple Approaches to STEM Education Programs

Objective: In this study, we investigated the implementation of project‐based learning (PBL) activities in four secondary science, technology, engineering, and mathematics (STEM) education settings to examine the impact of inquiry based instructional practices on student learning. Method: Direct classroom observations were conducted during the 2013–2014 school year in STEM Traditional Courses, a STEM Platform School, an Engineering Optional Program (EOP), and a Virtual STEM Academy (VSA) to measure teacher instructional practices (School Observation Measure) and student engagement (The Rubric for Student‐Centered Activities). Results: The four approaches to STEM education showed significant differences in their implementation of PBL, with the EOP and VSA having higher incidences of PBL activities. Additionally, higher‐level questioning strategies, higher‐order instructional feedback, and integration of STEM subject areas was absent or rarely observed. Conclusions: Components of PBL are missing in STEM education, in traditional and non‐traditional STEM courses. In‐service teachers may benefit from professional development that enhances their understanding of PBL activities to maximize student learning opportunities.     

Martingales and buffer overflow for the symmetric shortest queue model

Queueing Systems - A variant of the standard symmetric system of two parallel queues under the join-the-shortest-queue policy is introduced. Here, the shortest queue has service rate $$mu _1$$ ,...     

Hindered coarsening of a phase-separating microemulsion due to dispersed colloidal particles

The addition of sterically stabilized colloidal particles to a phase-separating microemulsion leads to dramatic changes in its demixing behavior, especially during the later stages. Our microemulsion is composed of reverse micelles of sodium dodecyl sulfate, pentanol, and water in a dodecane continuous phase which separates into micelle-rich and micelle-poor phases above a lower critical solution temperature. The poly(methyl methacrylate) particles preferentially partition into the less structured, micelle-poor phase. Nucleation of the minority phase or spinodal decomposition close to criticality continue to occur in the presence of particles, albeit with pronounced pretransitional clustering of particles when the micelle-poor phase is in the minority. The coalescence of micelle-poor droplets and the coarsening of micelle-rich domains are both strongly modified due to the presence of colloidal particles. We use our observations of the early stages of phase separation to understand these late stage changes.