Manganese-quinizarin-pyrazinecarboxylic acid mixed-ligand complex in aprotic media. Formation of a manganese(III)- semiquinone binuclear species

The monoanion of pyrazincarboxylic acid (PcA^–), the dianion of quinizarine (Qz⁼) and manganese(II) yield a soluble deep-blue complex in dimethylsulphoxide whose Mn^II:Qz⁼:PcA^– stoichiometry has been established as 1:1:1. This mixed-ligand complex is oxidized in two steps, each involving one equivalent of charge per complex present, as indicated by controlled-potential electrolyses done at +0.30V versus s.c.e. and +0.50 V versus s.c.e.. The association between the metal ion and the ligands prevail and the oxidations finally produce a dark-red complex which possesses the same stoichiometry as the original deep-blue species. In the latter the metal ion is present in oxidation state +3 with the quinizarine dianion as the corresponding semiquinone of the oxidized quinizarine. The monoanion of pyrazincarboxylic acid remains unchanged. Controlled-potential electrolysis at –0.20 V versus s.c.e. of a solution of the mixed-ligand complex indicates that it is binuclear, generating a Mn^II-Mn^IIImixed-valence species. The latter is, in turn, reduced at –1.60 V versus s.c.e. producing probably a mononuclear of manganese(II) species. If manganese(II) is combined with the semiquinone of quinizarine the metal ion exhibits themagnetic characteristic of manganese(III) and thesemiquinone is reduced to the quinizarine dianion, indicating that the mixed-ligand formed exhibits intramolecular charge-transfer. This is a good example of a binuclear species accumulating four oxidation equivalents after oxidation of both metal centers and the quinizarine ligands.     

On the growth kinetics and structure of the NiSi compound on silicon single crystals

Summary The growth kinetics of NiSi has been evaluated by depositing a Ni film on <111> and <100> Si substrates. X-ray diffraction and MeV⁴He⁺ backscattering have been used to identify the compounds formed and to measure their thicknesses. Carbon and oxygen impurities have been detected with Auger electron spectrometry with Ar sputtering. During thermal annealing Ni₂Si is formed first, while NiSi appears only after the total exhaustion of the Ni film. It has been found that, on both Si substrates, the thickness of NiSi increases with the square root of time. Different growth rates have been observed for the two different Si orientations. The activation energy in the (300÷370)°C temperature range is 1.83 eV for NiSi grown on <111> and 1.23 eV for NiSi on <100> Si. The two competing mechanisms of Ni diffusion through NiSi by grain boundary and by substitutional processes are correlated with the different microcrystalline structure of the NiSi silicide layers grown on different Si-oriented substrates.

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Riassunto Sono state studiate, con la diffrattometria ai raggi X e con la tecnica RBS di ioni⁴He accelerati ad energie dell'ordine del MeV, la formazione e la velocità di crescita del composto NiSi ottenuto per trattamento termico di uno strato sottile di Ni depositato su Si monocristallino <111> e <100>. La presenza di eventuali impurezze, tipo carbone e ossigeno, è stata osservata con la spettrometria Auger combinata con la rimozione catodica del materiale. Il primo composto che si forma dalla reazione Si−Ni è l'Ni₂Si. Successivamente l'NiSi cresce per la reazione dell'Ni₂Si con il Si e ciò avviene solo dopo che tutto il ni si è consumato nella formazione dell'Ni₂Si. Lo spessorex dell'NiSi cresce col tempot secondo una legge del tipo su entrambi i tipi di Si monocristallino. La velocità di crescita su Si <100> è però maggiore di quella su Si <111>. Sono diverse anche le energie di attivazione del processo di diffusione che determina la crescta dell'NiSi. Infatti, nell'intervallo di temperatura (300÷370)°C, si ottiene un'energia di attivazione di 1.83 eV per l'NiSi cresciuto su Si<111> e 1.23 eV su Si<100>. Dall'analisi ai raggi X si correla la diversa microstruttura dell'NiSi cresciuto su substrati di Si diversamente orientati con le cinetiche di crescita e con le energie di attivazione del processo. Due meccanismi di diffusione competitivi sono responsabili dei diversi risultati ottenuti sui due tipi di substrato monocristallino. Su Si <100> predomina la diffusione di atomi attraverso i bordi di grano dell'NiSi, mentre sul <111> predomina la diffusione per sostituzione successiva degli atomi del composto.

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Резюме Оценивается кинематика роста соединения NiSi, образованного посредством осаждения пленки Ni на подложках <111> и <100> Si. Используется дифракция ренттеновских лучей и обратное рассеяние МэВ-ных⁴He⁺ для идентификации полученных соединений и измерения их толщины. Примеси углерода и кислорода определяются с помощью спектроскопии Оже-электронов при Ar распылении. Во время термического отжига сначала овразуется Ni₂Si, тогда как NiSi появляется только после полного истошения пленки Ni. Обнаружено, что на обоих подложках <111> и <1000> Si толшина NiSi увеличивается как квадратный корень из времени. Для двух различных ориентаций Si наблюдаются различные скорости роста. Энергия активации в области температур (300÷370)°C составляет 1.83 эВ при вырашивании NiSi на <111> Si и 1.23 эв при вырашивании NiSi на <100> Si. Два конкурируюших механизма диффузии Ni через NiSi на границе зерна или в результате процессов замещения коррелируют с различными микрокристаллическими структурами слоев NiSi, выращенных на подложках кремния с различной ориестациеи.

     

Substituent Effects on Hydrogen Bonding in Watson–Crick Base Pairs. A Theoretical Study

We have theoretically analyzed Watson–Crick AT and GC base pairs in which purine C8 and/or pyrimidine C6 positions carry a substituent X = H, F, Cl or Br, using the generalized gradient approximation (GGA) of density functional theory at BP86/TZ2P. The purpose is to study the effects on structure and hydrogen bond strength if X = H is substituted by a halogen atom. Furthermore, we wish to explore the relative importance of electrostatic attraction versus orbital interaction in the above multiply hydrogen-bonded systems, using a quantitative bond energy decomposition scheme. We find that replacing X = H by a halogen atom has relatively small yet characteristic effects on hydrogen bond lengths, strengths and bonding mechanism. In general, it reduces the hydrogen-bond-accepting- and increases the hydrogen-bond-donating capabilities of a DNA base. The orbital interaction component in these hydrogen bonds is found for all substituents (X = H, F, Cl, and Br) to contribute about 41% of the attractive interactions and is thus of the same order of magnitude as the electrostatic component, which provides the remaining 59% of the attraction.     

Anthracycline-dependent heat-induced transition from positive to negative supercoiled DNA

The conformational stability of individual DNA topoisomers depends on the concentration of DNA intercalating drugs. To study the DNA-drug interaction, we used ethidium bromide (EtBr) and negative supercoiled pUC19 as a model system. The effects of two anthracyclines widely used in cancer therapy, daunorubicin (Dau) and doxorubicin (Doxo), and EtBr were compared. In spite of their different chemical structures and intercalation mode, all intercalating agents show similar effects on the conformational stability of supercoiled DNA. Our observations show that the studied intercalators have at least two main effects on the supercoiled DNA: (i) they decrease the level of negative supercoiling and, at certain concentrations, they may induce positive supercoiling in DNA; (ii) a temperature increase can cause a recovery of negative supercoiling in DNA. The conformational stability of plasmid DNA-drug complexes has been investigated by temperature gradient gel electrophoresis (TGGE). We demonstrate the suitability of TGGE for this purpose, because it offers a global view on DNA-drug complexes over a continuous range of temperature. Images of DNA plasmids adsorbed onto a substrate at different temperatures and drug concentrations were acquired by atomic force microscopy (AFM), allowing us to distinguish directly the conformation chirality assumed by the plasmid under different conditions confirming TGGE results. Our detection system allows to characterize unknown drugs and to determine their intercalating properties.     

Effect of Chemical Crosslinking on the Swelling and Shrinking Properties of Thermal and pH‐Responsive Chitosan Hydrogels

The ability to form a gel through the physical or chemical crosslinking of chitosan has been well documented. In an attempt to mimic biological systems, thermal and pH‐sensitive chitosan cylindrical hydrogels were produced by a combination of physical and chemical crosslinking processes. To this end, chitosan hydrogels prepared from alkali chitin were molded in cylinders and, once washed, were further crosslinked with glutaraldehyde at stoichiometric ratios, R (= [? CH?O]/[? NH₂]), of 1.61 and 3.22 × 10^?2. Variation in swelling as a result of stepwise changes in temperature between 40 and 2 °C at pH values of 7.0, 7.6, and 8.0 revealed that the system responds in markedly different manners dependent upon the pH. At pH 7.0, cooling from 40 to 2 °C results in contraction of the gel network structure. While raising the temperature from 2 to 40 °C leads to a rapid swelling response (i.e., ca. a twofold increase in the amount of solvent uptake). Subsequent cooling to 2 °C is accompanied by a new contraction cycle. At pH ≥ 7.6 the temperature dependence of the swelling–contraction behavior is exactly the opposite of that observed at pH 7.0. Very similar trends were observed for the gels at both degrees of crosslinking. The swelling–shrinking behavior observed in gels of pH ≥ 7.6, is similar in kind to that of uncrosslinked gels and is interpreted in terms of a lower critical solution temperature (LCST) volume phase transition, driven by hydrophobic association, presumably involving residual acetyl groups in the chitin. The results at pH 7.0 suggest that the slight ionization of the ? NH groups leads to destruction of the hydrophobic hydration thus effectively reversing the negative thermal shrinking.

Evolution of the swelling ratio, S, as a function of time and temperature for crosslinked chitosan hydrogels. Circles represent S values recorded at pH 7.0 and triangles those at pH 7.6.     

Application of SEM/EDX and metallographic techniques to the diachronic study (6th–18th century) of metallurgical materials found in archaeological excavations on the island of Ibiza (Spain)

This work is the archaeometric study of the different types of materials recovered in the same excavation or archaeological dig (which makes it possible to accurately date the remains found) on the island of Ibiza (Spain). The samples found belong only to the phase of iron forging and span a very wide historical period from the 6th to the 18th century, including the Islamic period. Scanning Electron Microscopy (SEM) is proposed to observe the images obtained of the surfaces of the samples, in order to study the topographical, morphological and microstructural characteristics providing information on the materials contained in the samples. It also permits chemical analysis of the elements in the sample using X-ray Microanalysis (SEM/EDX), which provides both qualitative and semi-quantitative information on the elements in the sample. The archaeometric results suggest that the sole use of the area studied was as a forge workshop over a long period of time. The slag studied show that no reduction or refining activities took place in the area. However, in this forge, pieces of other metals were also produced, especially bronze alloys, polymetallism being a common characteristic throughout history.     

Fast invisible neutrino decays

Any value for the lifetime of neutrinos decaying into invisible modes (another neutrino and a Goldstone boson) is possible in theories in which the lepton number is a spontaneously broken global symmetry with different charges for every family. There are two types of models. Only in one of them neutrinos could, in a natural way, be relevant for cosmology.     

Dual‐Genic Hybridization Sensor Employing Electrochemical Impedance Spectroscopy

A dual‐genic impedimetric sensor for DNA hybridization detection is proposed for the first time. Two different single‐stranded DNA probes (P₁=d(pA)₂₀ and P₂=d(pG)₂₀) were immobilized onto the graphite‐epoxy electrode surface and impedimetric measurements were performed. Hybridization experiments were carried out employing complementary targets (P₁^c=d(pT)₂₀ or P₂^c=d(pC)₂₀) alone, or combined (P₁^c+P₂^c), as well as noncomplementary DNA sequences. As from the direct observation of impedance spectra it was not possible to resolve the binary gene mixture, the use of artificial neural networks (ANN) was proposed for extracting significant information to get desired results.