Photoreduction of Terrigenous Fe‐Humic Substances Leads to Bioavailable Iron in Oceans

Humic substances (HS) are important iron chelators responsible for the transport of iron from freshwater systems to the open sea, where iron is essential for marine organisms. Evidence suggests that iron complexed to HS comprises the bulk of the iron ligand pool in near‐coastal waters and shelf seas. River‐derived HS have been investigated to study their transport to, and dwell in oceanic waters. A library of iron model compounds and river‐derived Fe‐HS samples were probed in a combined X‐ray absorption spectroscopy (XAS) and valence‐to‐core X‐ray emission spectroscopy (VtC‐XES) study at the Fe K‐edge. The analyses performed revealed that iron complexation in HS samples is only dependent on oxygen‐containing HS functional groups, such as carboxyl and phenol. The photoreduction mechanism of Fe^III‐HS in oceanic conditions into bioavailable aquatic Fe^II forms, highlights the importance of river‐derived HS as an iron source for marine organisms. Consequently, such mechanisms are a vital component of the upper‐ocean iron biogeochemistry cycle.     

Fast determination of reaction kinetic parameters with the use of microwave heating. Kinetics of decomposition of organic substances with nitric acid

A mathematical model for the kinetics of a chemical reaction under microwave irradiation in a close volume in the simplest approximation of the uniform temperature and pressure distribution in a system was proposed. The previously proposed dynamic method for determination of the activation energy of oxidation of organic substances with nitric acid was substantiated. According to this method, the activation energy is determined from the experimental exponential plot for the pressure in the nonisothermic system vs. inverse temperature of the reaction mixture. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1371–1375, June, 2005.     

New approach in the treatment of data from an acid–base potentiometric titration

Based on precise analysis of the acid-base equilibrium, a new approach in the treatment of experimental data from a potentiometric titration is proposed. A new general formula giving explicitly the relation V=f([H(+)]) is derived, valid for every acid-base titration, which includes mono- and polyfunctional protolytes and their mixtures. The present study is the first practical application of this formula for the simplest case, the analysis of one monofunctional protolyte. The collected mV data during the titration are converted into pH-values by means of an auto pH-calibration procedure, thus avoiding preliminary preparation of the measuring system. The mentioned pH-calibration method is applicable also in water-organic mixtures and allows the quantitative determination of sparingly soluble substances (particularly pharmaceuticals). The treatment of the data is performed by means of ready-to-use software products, which makes the proposed approach accessible for a wide range of applications.     

Quantification of refractory organic substances in freshwaters: further insight into the response of the voltammetric method

A recently published method for quantifying refractory organic matter (often referred to as humic substances) in freshwaters was applied to a wide range of International Humic Substance Society (IHSS) humic compounds in order to (i) gain a better understanding of the mechanism of the voltammetric response which is the basis of the analytical method and (ii) provide guidance on choosing the optimal standard to be used. At the same time, the sensitivity of the technique has been increased by switching from the pulse mode initially proposed to the square-wave mode. The results obtained show that (i) differences in adsorption onto the electrode rather than differences in complexation strength are responsible for the differences in the intensity of the signal obtained for the different humic compounds, (ii) carboxylate, N- and S-containing groups do not play a role in the voltammetric signal.     

Nature of the chromatographic process for inorganic substances on cellulose and silica gel thin layers

Summary The mechanism of interaction between inorganic substances and thin layers of microcrystalline cellulose and silica gel is considered. It is observed that the majority of the phenomena associated with the chromatographic behaviour of inorganic substances are not in agreement with the hypothesis of a partition mechanism on cellulose, nor with the ion-exchange mechanism on silica gel. It is concluded that the mechanism of interaction between inorganic substances and cellulose or silica gel is based on adsorption, due to electrostatic forces, primarily through hydrogen bonds between OH-groups of the adsorbent and basic ligands of inorganic complexes. The chromatographic process comprises a continual dynamic adsorption—desorption equilibrium of the complexes between the mobile phase and the stationary phase.     

Research in the method of large molecular calculations utilizing transferability of LMO

The localized molecular orbitals of some saturated hydrocarbons and their derivatives have been formed using ab initio method and M. P. [1–2] localization procedure. Two models, SLMO and ELMO , a set of parameters of LMO Fock matrix elements, and a technique called “Group Effect” are proposed. Based on these, we developed a procedure to simulate the ab initio calculations on large molecules. Some test calculations have been done. The results are compared with those of the ab initio method. In general, absolute errors of orbital energies are about 10^?3 a.u., and the relative errors of total energies are about 10^?4. For the original applications, we applied this procedure to some large systems of alkane and their derivatives as well as three Crown-ether compounds. Satisfactory results are obtained.     

Cross‐validation in PCA models with the element‐wise k‐fold (ekf) algorithm: theoretical aspects

Cross‐validation has become one of the principal methods to adjust the meta‐parameters in predictive models. Extensions of the cross‐validation idea have been proposed to select the number of components in principal components analysis (PCA). The element‐wise k‐fold (ekf) cross‐validation is among the most used algorithms for principal components analysis cross‐validation. This is the method programmed in the PLS_Toolbox, and it has been stated to outperform other methods under most circumstances in a numerical experiment. The ekf algorithm is based on missing data imputation, and it can be programmed using any method for this purpose. In this paper, the ekf algorithm with the simplest missing data imputation method, trimmed score imputation, is analyzed. A theoretical study is driven to identify in which situations the application of ekf is adequate and, more importantly, in which situations it is not. The results presented show that the ekf method may be unable to assess the extent to which a model represents a test set and may lead to discard principal components with important information. On a second paper of this series, other imputation methods are studied within the ekf algorithm. Copyright © 2012 John Wiley & Sons, Ltd.     

A highly membrane-active peptide in Flock House virus: implications for the mechanism of nodavirus infection.

BACKGROUND: Nodaviruses are among the simplest animal viruses, and are therefore attractive systems for deconvoluting core viral processes such as assembly, infection and uncoating. Membrane translocation of the single-stranded RNA genome of nodaviruses has been proposed to be mediated by direct lipid-protein interactions between a post-assembly autocatalytic cleavage product from the capsomere and the target membrane. To probe the validity of this hypothesis, we have synthesized a 21-residue Met-->Nle (norleucine) variant of the amino-terminal helical domain (denoted here as gamma1) of the cleavage peptide in Flock House nodavirus (FHV) and studied its ability to alter membrane structure and function. RESULTS: The synthetic peptide gamma1 increases membrane permeability to hydrophilic solutes, as judged by fluorescence experiments with liposome-encapsulated dyes and ion-conductance measurements. Furthermore, peptide orientation and location within lipid bilayers was determined using tryptophan-fluorescence-quenching experiments and attenuated total reflectance infrared spectroscopy. CONCLUSIONS: The helical domain of the FHV cleavage product partitions spontaneously into lipid bilayers and increases membrane permeability, consistent with the postulated mechanism for viral genome translocation. The existence of a membrane-binding domain in the FHV cleavage sequence suggests peptide-triggered disruption of the endosomal membrane as a prelude to viral uncoating in the host cytoplasm. A model for this interaction is proposed.     

Non-traditional pathways of extraterrestrial formation of organic compounds

The mechanisms of solid-phase reactions that have been experimentally and theoretically studied during recent decades and can be directly related to the formation of organic substances in space and their delivery to Earth and to the problems of prebiotic evolution are considered. Among these mechanisms are molecular tunneling (hypothesis of the cold prehistory of life), polycondensation of solid monomers by shock waves (problem of the delivery of organic substances to Earth by meteorites), thermal and thermal-wave explosions. and oscillations of temperature and radical concentrations in small cold particles under radiation exposure, mechanochemical explosions and autowave propagation of chemical reactions due to the positive feedback between fragile destruction of solids and reactions at freshly formed surfaces. Written on the basis of the report at the International Conference “Chemical Physics at the Threshold of XXI Century,” April 16–19, 1996, Moscow. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 409–417, March, 1997.     

Nouvelle méthode de dosage automatique de substances oxydantes ou réductrices par potentiométrie différentielle : Partie I. Étude théorique

A new method is proposed for routine determinations of oxidizable and reducible substances. A mathematical development and criticism of the method of Schierjott has led to a fundamental modification of the earlier experimental conditions (ratio of the oxidized/reduced forms of the reagent at the start and end of the redox reaction). This improvement allows a considerable increase in the sensitivity of the method and is particularly advantageous for the determination of substances of low reactivity, e.g. carbohydrates. Other advantages are a reduction in the quantity and thus cost of the necessary reagent, and a decreased risk of precipitation of the substances involved. The mathematical treatment of the proposed version allows a rigorous theoretical comparison of the two methods. An evaluation of the quality of the approximations introduced into both methods is also proposed, based on the calculation of the deviation (absolute and relative differences) of approximated values from theoretical values. The final part of the work deals with optimization of the different principal parameters, i.e. concentration of the reagent, temperature and reaction time.     

Particle size correlations in brownian agglomeration. Closure hypotheses for product density equations

In a previous paper the authors have demonstrated inaccuracies in the population balance equation, as applied to agglomerating particulate systems, which arise due to an inherent assumption regarding the statistical independence of particle pairs. When this assumption is removed, an intractable hierarchy of product density equations results. In this paper a closure hypothesis for the product density equations is proposed, evaluated against alternatives, and used in an approximate solution to the product density equations. The closure hypothesis is shown to improve significantly upon the population balance equation which is itself the result of the simplest closure.     

Risk Analysis in Transport and Storage of Monomers: An Accident Investigation

Summary: In this work the analysis of an accident in transport of dangerous goods is proposed. The objective of this study is to contribute to the determination of the causes that brought about the self-polymerization of commercial divinylbenzene (DVB 63%) contained in an iso-container, in order to prevent this accident to happen in the future. Time and conditions during transport and storage of monomers (in particular the storage temperature) are very important factors, that affect the self-polymerization aptitude of these substances. To stabilize this monomer, usually 4-tert-Butylcatechol (TBC) is added at a level of 900–1200 ppm by weight to act as an inhibitor to prevent the self-initiated autopolymerization of the material. In particular one hypothesis has been investigated: probably the quantity of oxygen in the tanker was insufficient to activate properly the inhibition mechanism of TBC. From this consideration the self-polymerization of DVB and the inhibition mechanism of TBC have been studied as a function of temperature and monomer exposure to air. Different calorimetric techniques have been applied, in particular Differential Scanning Calorimetry and Adiabatic Calorimetry, to investigate the causes of the accident.     

Chromatographic Identification of Cyclohexane Chlorination Products by an Additive Scheme for the Prediction of Retention Indices

Identification of the products of free-radical chlorination of cyclohexane seems to be a complex analytical problem due to the following principal reasons: (1) the available reference information (both standard mass spectra and gas chromatographic retention indices, RI) is strongly restricted to the data for only few simplest congeners, (2) mass spectra of isomeric chlorinated derivatives are practically indistinguishable, and (3) the number of isomers increases with the growing number of chlorine atoms in the molecule until six. To solve this problem, the modified additive scheme for precalculation of the RIs of chlorinated cyclohexanes on standard non-polar polydimethyl siloxane stationary phases was proposed and used. This approach is based on the data for congeners with fewer number of chlorine atoms in the molecule; the principal feature is its applicability to a diastereomers. The final set of both experimental and precalculated RI values permits us to identify unambiguously more than 20 products of cyclohexane chlorination in reaction mixtures. Different modes of the quality control of evaluated retention indices are discussed.     

Imaging fingerprinting of excitation emission matrices

The spectral fingerprinting of the excitation emission matrix (EEM) of fluorescent substances is demonstrated using polychromatic light sources and tri-chromatic image detectors. A model of the measured fingerprints explaining their features and classification performance, based on the polychromatic excitation of the indicators is proposed.Substantial amount of spectral information is retained in the fingerprints as corroborated by multivariate analysis and experimental conditions that favor such situation are identified.In average, for five different substances, the model shows a fitting goodness measured by the Pearson's r coefficient and the root mean square deviation of 0.8541 and 0.0247 respectively, while principal component classification patterns satisfactorily compare with the EEM spectroscopy classification and respectively explain 96% and 93% of the information in the fist two principal components.The measurements can be performed using regular computer screens as illumination and web cameras as detectors, which constitute ubiquitous and affordable platforms compatible with distributed evaluations, in contrast to regular instrumentation for EEM measurements.     

Multicomponent Adsorption of Pesticides onto Activated Carbon Fibers

The adsorption equilibria of pesticides and metabolites (atrazine, deethylatrazine, deisopropylatrazine and simazine) are studied onto activated carbon fibers –ACF– with a broad pore size distribution (32% mesopore volume, 68% micropore volume). Mono-and multi-component isotherms have been determined for low concentrations, from 0.23×10⁻⁶ to 9.52×10⁻⁶ mol L⁻¹. Single solute isotherms, modeled by Freundlich and Langmuir models, tend to prove the influence of the adsorbate's solubility in the adsorption capacity of activated carbon fibers. Binary solute isotherms confirm the strong influence of pesticide solubility on the competitive adsorption mechanism: the competition is higher in the case of adsorbates of different solubilities (atrazine and DEA or DIA for example). Multicomponent experimental data were modeled by extended Langmuir-based equations and the Ideal Adsorbed Solution theory. Whereas the first ones failed to model accurately binary adsorption due to restrictive hypothesis, the IAS model showed a good agreement between experimental and predicted data. It emphasised also the difficulty in satisfying the hypothesis of the model in the case of highly adsorbed compounds. Finally, the simultaneous adsorption of atrazine and NOM (in a natural water, DOC = 18.2 mg L⁻¹) shows no adsorption competition effects between natural organic matter and atrazine. This is due to the presence of secondary micropores (0.8–2 nm) and mesopores in the ACF, which limit a pore blockage phenomenon by NOM.     

pH区带逆流色谱研究进展

pH区带逆流色谱是在逆流色谱基础上发展起来的一种新型制备型分离技术。利用物质解离常数和疏水性的不同进行分离,具有分离量大、分离效果好等优点,应用领域比较广泛;简单介绍了该技术的分离原理及分类,并对该技术在氨基酸衍生物、肽衍生物、染料、生物碱和异构体等分离纯化中的应用及其改进方向进行了较为全面的总结,展望了其发展前景。     

Electron mechanism of nonequilibrium molecular vibration excitation in biological systems

The electron mechanism of the nonequilibrium vibration excitation is considered in multiatom molecules with the condition that the bottleneck effect takes place. The quantum-statistical theory states that if the simplest enzyme reaction is proposed on the basis of the vibrational energy recuperation idea, the vibrons emitted in producing the free product and enzyme activate the first stage of the formation of the Michaelis–Menten complex and the multiphonon feedback mechanism is realized. Resulting solutions are applied to explain some biochemical and genetic effects.     

Migration of the mobile phase and zones of separated substances in flat-bed chromatography

It is proposed to extend the area of application of circular separation of substances in flat-bed chromatography by a wider use of its intermedial modifications. Equations for calculating the basic parameters of migration of the mobile phase and zones of separated substances are given. The known equations are shown to be particular cases of the ones proposed.     

About entangled networks of worm-like micelles: a rejected hypothesis

We report new results from small-angle neutron scattering ond ₁₂-cyclohexane/lecithin/water micellar solutions performed as a function of the water content (w ₀), temperature (T) and dispersed phase volume fraction (). The data from dilute samples are interpretable in terms of the existence of giant cylindrical reverse micelles and are well fit with a core-shell model (that provides the micelle structure and dimensions) with values of 28 and 45 Å for the inner core and the outer shell radii, almost independent on temperature and concentration. Such a result could appear consistent with the current idea that worm-like micelles are living polymers. On the contrary, the appearance of a sharp interference maximum at high concentrations (>0.15) suggests arguments against the current hypothesis of an entangled network of giant flexible cylinders. Further arguments against the current hypothesis are given by the close similarity between the above described results and those from free of water micelles (for sure not cylinders). All the data are well fitted in terms of a unique model taking into account the micellar form factor plus a hard sphere structure factor. The data analysis suggests a micellar size distribution determined by the competition between concentration and interaction effects on which temperature plays not a minor role. Following our results, the current hypothesis of a gel structure in terms of an entangled network can be assumed as wrong and some caution has to be taken in assuming wormlike micelles as living polymers.     

Electron correlation: the many-body problem at the heart of chemistry

The physical interactions among electrons and nuclei, responsible for the chemistry of atoms and molecules, is well described by quantum mechanics and chemistry is therefore fully described by the solutions of the Schr?dinger equation. In all but the simplest systems we must be content with approximate solutions, the principal difficulty being the treatment of the correlation between the motions of the many electrons, arising from their mutual repulsion. This article aims to provide a clear understanding of the physical concept of electron correlation and the modern methods used for its approximation. Using helium as a simple case study and beginning with an uncorrelated orbital picture of electronic motion, we first introduce Fermi correlation, arising from the symmetry requirements of the exact wave function, and then consider the Coulomb correlation arising from the mutual Coulomb repulsion between the electrons. Finally, we briefly discuss the general treatment of electron correlation in modern electronic-structure theory, focussing on the Hartree-Fock and coupled-cluster methods and addressing static and dynamical Coulomb correlation.