Using isothermal titration calorimetry to real-time monitor the heat of metabolism: a case study using PC12 cells and Aβ(1-40)

Isothermal titration calorimetry (ITC) is one of the most powerful means for direct determination of thermodynamic information associated with most physiochemical and biological processes. The deposition and aggregation of β-amyloid (Aβ) on cell membranes was considered as one of the primary factors in having Alzheimer's disease (AD). Recently, a growing body of evidence has suggested that plasma membrane could accelerate the accumulation of Aβ on the plasma membranes. However, the mechanism of AD is still a controversial issue. This study provided a biothermodynamic approach to real-time monitor the heat of metabolism involved in the co-incubation of PC12 cells and Aβ(1-40) by ITC. The effects of Aβ conformation and concentration of oligomeric Aβ on cytotoxicity were successfully distinguished by ITC. This approach with rapid and direct measurement may provide not only real-time information for the effects of Aβ species on living cells but also a platform for the screening of drug candidates for AD.     

A critical study of the application of ultraviolet spectroscopy to the self-association of adenine,adenosine and 5′-AMP in aqueous solution

UV spectra of adenine, adenosine and 5′-AMP in aqueous solution have been measured over the concentration range 1 × 10⁻⁶−5 × 10⁻² M. The apparent molar absorptivity of these compounds changes upon concentration, showing two hypochromic effects at c < 5 × 10⁻³ M and c &>; 5 × 10⁻³ M, respectively, which may be explained in terms of self-association.A method for calculating self-association constants from these experimental data is developed, based on an association model in which the first hypochromic effect is interpreted in terms of formation of dimers, with an equilibrium constant K₂, and the second effect is interpreted in terms of formation of polymers, with an equilibrium constant K_n. The value of K₂ is of the order of magnitude of 10⁴ for the three compounds. The value of K_n is dependent on the model chosen for the analysis of the second effect, having an order of magnitude of 10².The features of the self-association model are discussed, as well as the method for calculating self-association parameters from experimental data.     

What’s that yellow powder? A nuclear forensic case study

A challenge to the incorporation of ⁹⁹Tc into a glass matrix is that ⁹⁹Tc is volatile at vitrification temperatures. Understanding how this volatilization occurs requires knowledge of the multiple chemistries which Tc may take during vitrification. This paper presents an overview of how the localized chemistry of ⁹⁹Tc has been determined in a series of ⁹⁹Tc-bearing glasses by chemometric analyses of X-ray absorption near edge spectra (XANES). Linear combination fitting and principal component analysis of the glasses’ XANES spectra suggested that the glasses contained 3–4 chemically distinct Tc environments. The identity of the detected chemistries were pertechnetates, (Tc(VII) as isolated oxyanions, or locally coordinated by Na or K), or isolated Tc(IV) species. The linear combination fitting distribution of local Tc sites agrees with reanalyzed Raman spectra, suggesting that as targeted KTcO₄ incorporation increases, a significant ion exchange takes place, and speciation in the glass changes to favor Tc(VII) formation, specifically NaTcO₄. Based on the statistical suggestion that not all Tc environments are accounted by the available standards, a new mechanism for the behavior of Tc during vitrification is proposed.     

Development of a natural ingredient – Natural preservative: A case study

Lately, the cosmetic and personal care market has been more and more driven toward natural ingredients by the rising consumers' awareness about personal health and safety and their will for safer cosmetics free of harmful chemicals. Preservatives are no exception to the rule: evidence or suspicion of the toxicity of certain synthetic preservatives that have been around for decades pushed the cosmetic industry forward to seek for natural alternatives, as the selection of natural preservatives already available is quite limited. Sourcing active metabolites and developing new natural ingredients are long-term procedures that are thoroughly described in the present paper, via the example of the design of a natural preservative based on the Santolina chamaecyparissus extract, and of the assessments of its preservative effectiveness.     

Do spin state changes matter in organometallic chemistry? A computational study

Spin changes occur often in organometallic chemistry, and their effect on kinetics is not well understood. We report computations on the singlet and triplet potential energy surfaces of several processes of this type and show that the topology of the individual surfaces, as well as of the crossing regions between them, can be used to rationalize the observed reactivity in all cases. In particular, the slow addition of dihydrogen to W[N(CH(2)CH(2)NSiMe(3))(3)]H (Schrock, R. R.; Shih, K. Y.; Dobbs, D. A.; Davis, W. M. J. Am. Chem. Soc. 1995, 117, 6609) is shown to be a "spin-blocked" reaction with a high barrier due to the crossing between reactant triplet and product singlet surfaces. In contrast, addition of CO to TpCo(CO) (Detrich, J. L.; Reinaud, O. M.; Rheingold, A. L.; Theopold, K. H. J. Am. Chem. Soc. 1995, 117, 11745) is fast because the triplet and singlet surfaces cross at low energy. Particular care is taken to use DFT methods which are in adequate agreement with experimental and high-level computational energetics for these systems.     

Do proteins at low temperature behave as glasses? A single-molecule study

We have recorded long spectral diffusion trajectories from individual LH2 pigment-protein complexes from the purple bacterium Rhodobacter sphaeroides at 1.4 K. From these data, the spectral cumulants of the absorption lines of individual, protein-embedded BChl a pigments have been evaluated. It appears that the first and second cumulants cannot be described by the predictions of the well tested standard two-level system (TLS) model for spectral diffusion in glasses. The results of the present study clearly show that there is a fundamental difference between the relaxation behavior of our test protein and that of glasses.     

Incurred sample reanalysis of cefuroxime in rabbit ocular tissues—A case study

In this paper, we present the incurred sample reanalysis (ISR) data for cefuroxime in various ocular tissues of rabbits. Based on the cefuroxime concentration vs. time profile in various ocular tissues, three chosen time points enabled ISR assessment. Cefuroxime was quantitated in the ocular tissues using a published liquid chromatography–electrospray ionization/tandem mass spectrometry method operated under the multiple reaction-monitoring mode in positive ion mode. Regardless of the ocular tissue, the linearity range was 12.7–2760 ng/ml with a correlation coefficient (r²) of ≥0.996. All of the ISR samples representing various ocular tissues met the acceptance criteria. To the best of our knowledge, this is the first report showing the ISR of ocular tissues in any species.     

Quantitative aspects of analyzing small molecules – monitoring singly or doubly charged ions? A case study of ximelagatran

Precision, reproducibility and lower limit of quantitation (LLOQ) are important characteristics of a quantitative method. We have investigated these properties for Ximelagatran (Xi), which has a high tendency to form doubly charged ions in electrospray ionization (ESI), by studying the percentage of doubly charged species formed when varying the formic acid (FA) concentration, analyte concentration, amount of organic modifier and flow rate. It was found that the percentage of [Xi + 2H]²⁺ can be controlled to be more than 90% or less than 10% by varying the amount of FA present, and that the change between these values is dramatic. Furthermore, the percentage of [Xi + 2H]²⁺ formed decreases with increased analyte concentration and increased flow rate. No apparent relationship with the amount of organic modifier was found. The results have the implication that, by carefully controlling the selected parameters, the LLOQ, precision and reproducibility can be improved. We have compared the fragmentation of the singly and doubly charged species and concluded that the [Xi + 2H]²⁺ ion is more inclined to undergo fragmentation than [Xi + H]⁺. As a consequence, unusual instrumental settings had to be used for the experiments. The fragmentation patterns are to a great extent similar, but the doubly charged species is more inclined to generate low‐mass product ions. Copyright © 2010 John Wiley & Sons, Ltd.     

How to estimate moments and quantiles of environmental data sets with non-detected observations? A case study on volatile organic compounds in marine water samples

Concentrations of 27 priority volatile organic compounds were measured in water samples of the North Sea and Scheldt estuary during a 3-year monitoring study. Despite the use of a sensitive analytical method, a number of data were censored. That is, some concentrations were below the decision limit or critical level defined by IUPAC. To characterize the observed measurement results, an attempt was made to identify an appropriate procedure to compute summary statistics for the censored data sets. Several parametric and robust parametric approaches based on the maximum likelihood principle and probability-plot regression method were evaluated for the estimation of the mean, standard deviation, median and interquartile range using three uncensored analytes (1,1,2-trichloroethane, tetrachloroethene and o-xylene) from the monitoring survey. Performance was assessed by artificially censoring the observed concentrations and estimating moments and quantiles at each censoring level. Results showed that methods with the least distributional assumptions, such as the robust bias-corrected restricted maximum likelihood method, perform best for estimating the mean and standard deviation, while both parametric and robust parametric techniques can be used for quantiles. Hence, summary statistics could be estimated with little bias (5-10%) up to 80% of censoring for the data sets employed in this study.     

Computational study of the amination of halobenzenes and phenylpentazole. A viable route to isolate the pentazolate anion?

Amination of halobenzenes, which proceeds via the benzyne intermediate (1), has been studied using quantum chemical methods. The computational data are in agreement with experimentally observed trends in reactivity and provide a qualitative explanation for the observed hydrogen isotope effects. To investigate if this is a viable way to isolate the pentazolate anion (2), the reactivities of the halobenzenes have been compared to phenylpentazole (3). The reaction energetics for phenylpentazole become favorable after complexation with Zn(2+).     

How to determine IR molar absorption coefficients of co-adsorbed species? Application to methanol adsorption for quantification of MgO basic sites

Methanol adsorption on MgO samples with different morphologies is studied by infrared (IR) spectroscopy coupled to volumetry or thermogravimetry measurements to probe qualitatively and quantitatively the acid-base paired sites. The molar absorption coefficients of ν(OC) vibration of non-dissociated methanol, type I and type II methoxy species are determined by analyzing data obtained under specific adsorption conditions (ε(ND)=2.5 cm μmol(-1); ε(I)=ε(II)=6.1 cm μmol(-1)). Thanks to these results, the amounts of different adsorbed methanol species are evaluated. These various species are formed on surface sites presenting different topologies. Hence, the IR method is the only one allowing us to both discriminate and quantify the defects on the MgO surfaces, in terms of concentrations of convex and concave defective zones. This study reveals that sol-gel preparation leads to a MgO surface presenting a greater amount of concave defective zones than precipitated MgO.     

A critical study of the hydrogen position in the crystal lattice of boehmite, γ-AlO(OH)

The relative intensities of i.r. absorption bands due to proton vibrations in boehmite samples of different origin are analysed and interpreted in terms of slight modifications of the hydrogen positions in the crystal lattice. There seems to be a linear relation between two angles characterizing the geometry of the hydrogen bond. The structural deformations are correlated with impurity content of boehmite samples.     

Do computed crystal structures of nonpolar molecules depend on the electrostatic interactions? The case of tetracene

We present a strategy for comparing the global properties of competing potential models. By systematically sampling the potential energy surface of crystalline tetracene, we assess how the number, energy and structure of its minima are modified by switching on (or off) the Coulombic interactions. The increased complexity of the Coulombic potential leads to a more "rugged" potential energy surface with a larger number of minima, but the effect is not large. In fact, we find a subset of minima stable only in presence of the Coulombic interactions, a smaller subset stable only in their absence, and a large majority stable in both cases. Among these, there is a very good, but not perfect, correlation between the energies and the structures computed with and without the electrostatic interactions. Although electrostatic interactions play a role even in a rigid nonpolar molecule such as tetracene, they are not as crucial as often believed, because altering the electrostatic model (or switching it off completely) leads, in most cases, to equivalent results.     

Can DFT methods correctly and efficiently predict the coordination number of copper(I) complexes? A case study

The coordination number of various experimentally known Cu(I) compounds is studied using density functional theory. Various basis sets are tested, aiming to establish a reliable level for prediction of the coordination number of these and other Cu(I) complexes. It is found that most levels exhibit correct trends, namely, the bulkier ligands demonstrate larger preference for coordination of two ligands. Proper absolute values are obtained when dispersion corrections are also included in the calculations. It is concluded that the fairly small modified 6‐31+G* basis set due to Pulay represents a good compromise between accuracy and efficiency, followed by Balabanov and Peterson's all‐electron aug‐cc‐pVDZ basis set. The overall energy is decomposed into various components whose relative contribution to the overall tendency of forming a complex with a particular coordination is examined. It is shown that two opposing contributions play a major role: the interaction energy of the ligand being added and the deformation energy of the copper's coordination sphere prior to the ligand addition. The former being a stabilizing contribution, leads to higher coordination numbers while the later, a destabilizing contribution, is shown to favor lower coordination numbers. © 2009 Wiley Periodicals, Inc. J Comput Chem 2010     

A computational study of tetrahedrene: strained alkene or dicarbene?

A computational study of tetrahedrene at the RCCD/cc-pVDZ level led to a singlet-state structure with a lowest energy vibrational mode of 660 cm(-1). The corresponding triplet state was found to be ca. 37 kcal/mol lower in energy than the singlet state. The heat of formation of the singlet state was estimated to be 270 kcal/mol. An isomeric singlet bicyclic dicarbene bis-bicyclo[1.1.0]cyclobutylidene was found to be approximately 94 kcal/mol lower in energy than tetrahedrene.