Chemical aspects of penicillin allergy. The direct penicilloylation of epsilon-amino groups by penicillin at pH 7.4

The aminolysis of penicillins by ε-amino groups at pH 7,4 is demonstrated. It is shown that penicillenic acid, formed spontaneously under these conditions from penicillins, does not contribute significantly to the penicilloylations observed. Penicillin therefore can directly form penicilloylamides under physiological conditions.     

Aspects of formation of the D-penicillamine-antigenic determinant from penicilloyl compounds

The formation of D -penicillamine-L -cysteine mixed disulfide from benzylpenicilloic acid, benzylpenilloic acid and benzylpenicilloyl amide derivatives in L -cysteine suspensions at pH 7,6 and 37° was studied. The reaction is faster than direct penicilloylation of proteins known to be a route to penicilloyl antigenic determinants. The production of S-bound penicillamine on protein from penicilloyl compounds must therefore definitely be considered as a potential antigenforming step. The reaction may be partly if not fully blocked by acylation of the thiazolidine nitrogen of penicilloyl compounds. When formylation is applied a considerable reduction of the capacity of penicilloyl antigenic determinants to interact with anti-penicilloyl antibody is noted. A D -penicillamine-specific test antigen was prepared by binding D -penicillamine via thioether links to a partly succinylated poly-L -lysine. Clinical test reactions elicited with this conjugate and with penicilloic acid cannot be well correlated. Penicilloic acid probably detects reactions of undefined specificity in addition to D -penicillamine-specific ones.     

Chemical pattern formation in gels

Precipitate forming chemical reactions have been studied in chemically crosslinked poly(vinyl alcohol) gel medium. One of the reactive components was incorporated into the gel, the other was allowed to diffuse into the swollen network. This reaction-diffusion process often results in pattern formation which can occur intermittently in terms of time and space or both. Experiments were carried out in order to investigate the effect of various factors (crosslinking density, swelling degree as well as the concentrations of the outer and inner electrolyte). The following precipitates were intensively studied: Mg(OH)₂, Cr(OH)₃, PbCl₂. Three basically different morphologies (Liesegang band formation, moving bands and tree-like structure) have been observed and investigated. One- and twodimensional computer simulations were carried out in order to establish the influence of various factors on the precipitation.     

The direct synthesis of methylchlorosilanes: New aspects concerning its mechanism

New results are given regarding the mechanism of the chemical process of copper alloyed silicon with methyl chloride (the `direct process'). As indicated by Photo-EMF measurements, carried out with doped silicon samples the reactivity of silicon significantly depends on the type of the doping with elements like phosphorus (n-type) tin, boron, indium (p-type). In-situ trapping experiments with 2,3-dimethylbutadiene are consistent with the creation of silylene intermediates SiMeCl and SiCl₂ . Theselectivity of their competitive insertion steps can be controlled by the doping type and concentrations of the doping elements, especially the phosphorus/tin ratio criterion. n-Type doping favors the silylene insertion into the C-Cl bond due to the electronic silylene stabilization on the silicon surface. In case of p-type doping silylene insertion into Si-Cl bond is favored more intensively leading to the formation of disilanes.     

Chemical and pharmacological aspects of capsaicin

Capsaicin is a unique alkaloid found primarily in the fruit of the Capsicum genus and is what provides its spicy flavor. Generally extracted directly from fruit, high demand has driven the use of established methods to increase production through extraction and characterization. Over time these methods have improved, usually be applying existing techniques in conjunction. An increasingly wide range of potential applications has increased interest in capsaicin. Especially compelling are the promising results of medical studies showing possible beneficial effects in many diseases. Capsaicin's pungency has limited its use in clinical trials to support its biological activity. Characterization and extraction/ synthesis of non-pungent analogues is in progress. A review is made of capsaicin research focusing mainly on its production, synthesis, characterization and pharmacology, including some of its main potential clinical uses in humans.     

Chemical-garden formation, morphology, and composition. II. Chemical gardens in microgravity

We studied the growth of metal-ion silicate chemical gardens under Earth gravity (1 g) and microgravity (μg) conditions. Identical sets of reaction chambers from an automated system (the Silicate Garden Habitat or SGHab) were used in both cases. The μg experiment was performed on board the International Space Station (ISS) within a temperature-controlled setup that provided still and video images of the experiment downlinked to the ground. Calcium chloride, manganese chloride, cobalt chloride, and nickel sulfate were used as seed salts in sodium silicate solutions of several concentrations. The formation and growth of osmotic envelopes and microtubes was much slower under μg conditions. In 1 g, buoyancy forces caused tubes to grow upward, whereas a random orientation for tube growth was found under μg conditions.     

Restricted Hartree–Fock approximation. II. Computational aspects of the direct minimization procedure

A RHF energy minimization procedure based on the treatment outlined in Part I of this series of articles is presented. Test calculations performed on several closed- and open-shell systems show that the present procedure is definitely superior to the conventional SCF methods. In particular, the convergence of this procedure is ensured, the rate of convergency is high, and the computational cost of each cycle is low.     

Some aspects of hydrate formation and wetting

Experimental observations of gas hydrate formation have shown that, in the initial nucleation and crystallization process, water-oil emulsions may be generated, destabilized or even inverted. These phenomena are consistent with the effects of particles on emulsions. In this work we relate experimental observations of hydrate formation to the phenomenon of wettability. It is shown that details of hydrate wetting are important for both the morphology and the kinetics of the formed hydrates. For the cases of hydrate lenses and spheres, it is shown that the various wetting states can be illustrated and analyzed by using wetting diagrams. Metastability is a function of the surface energies of the hydrate formation, i.e., the wetting state, and it is shown that in some cases metastability vanishes, and thus hydrates nucleates instantly at all positive driving forces. The magnitude of buoyancy and turbulence forces acting on a hydrate sphere are compared to the capillary force and it is concluded that capillary energy dominates when the hydrate spheres is less than 1 mm.     

Theoretical and practical aspects of acid-base determinants by spectrophotometric flow-injection analysis

A simple mathematical treatment is applied to the instantaneous acid-base equilibria prevailing at the peak maxima when reagents containing acid-base indicators are used for spectrophotometric flow-injection determinations of acids and bases. Equations are derived relating the absorbance, the fraction of the relevant acidic or basic form of the indicator and the rate of change of either form with the initial concentration of the analyte and other parameters that influence these determinations. Based on these relationships, criteria are set for the choice of reagent/indicator combinations likely to yield linear absorbance/concentration dependence for general and special cases. These considerations simplified the design of experimental work. Suitable conditions are described for the determination of hydrochloric, phosphoric and acetic acids and for sodium hydroxide and ethanolamine. Very weak acids and bases and concentrations lower than 0.01 N were not investigated, because they were not predicted to conform with the criteria for linearity presented. Least-squares treatment of the data obtained indicated a relative standard deviation of 0.5% from linear behavior, for favorable cases, over a reasonably wide concentrations range. Short-term standard deviations were 0.8% or better; long-term deviations, over several hours, were 1–2%. Highest throughput of samples examined was 144 h^?1.     

Chemical and pharmacological aspects of the genus Cecropia

The Cecropia genus consists of about 60 species distributed throughout Latin America, mostly in Brazil. These species are widely used in traditional medicine to treat cough, asthma, bronchitis, high blood pressure, inflammation, heart disease, and as a diuretic. In recent years, there has been an increase in research on Cecropia species, with several phytochemical and pharmacological reports. Concerning its chemical composition, C-glycosylflavonoids and proanthocyanidins have been described as the main constituents of C. glaziovii, C. hololeuca and C. pachystachya, while terpenoids and steroids have been reported in several species, including C. adenopus and C. pachystachya. Among the pharmacological properties described for the genus, the most frequently reported are hypoglycemic activity for C. obtusifolia and C. peltata, and hypotensive and central nervous system activity for C. glaziovii. The present review compiles the information available on this genus because of its ethnopharmacological relevance and the potential therapeutic uses of these species.     

Chemical aspects of amine oxidation by flavoprotein enzymes

The mechanism of amine oxidation by flavoprotein enzymes is critically analysed through analysis of available experimental data. The review assesses available evidence for radical and polar mechanisms, drawing on data from model reactions, kinetic and spectroscopic approaches, structural and mutagenesis studies. The review focused on studies of mammalian monoamine oxidases and the bacterial enzymes trimethylamine dehydrogenase, monomeric sarcosine oxidase and dimethylglycine oxidase.     

Chemical analysis of surface hydrocarbons in fireflies by direct contact extraction and gas chromatography-mass spectrometry.

We characterized three Japanese firefly species (Luciola lateralis, Luciola cruciata, and Lucidina biplagiata) and three North American firefly species (Lucidota atra, Photuris lucicrescens, and Photuris cinctipennis) based on their surface hydrocarbons. The analysis of firefly extracts by gas chromatography-mass spectrometry (GC-MS) revealed clear differences in the chromatographic profiles and mass spectra. Each firefly could be distinguished by its GC-MS profile. A major difference was observed between Japanese fireflies and North American fireflies. Among the North American fireflies, non-luminous fireflies, Lucidota atra, showed much more complicated GC-MS profile than those of luminous fireflies, Photuris lucicrescens and Photuris cinctipennis.     

Mathematical modelling of laser-induced particulate formation in direct solid microanalysis

The mechanism of laser-assisted particulate sampling was investigated by means of theoretical modelling. Bimodal particle size distribution was described as nanosized nuclei condensation in an expanding laser plume, and microsized droplet ejection in a hydrodynamically instable melt pool. Characteristics of the ambient gas turned out to be important in the formation of both size classes. Microparticle formation was found to be a process sensitive to local conditions (deterministic chaos). The model was applied to the case of ablation of silicon. Implications for the use of lasers as microsampling probes will be discussed throughout the article, as well as the impact on different solid sample classes.