Effect of Base–Acid Properties of Mixtures of Ethanol with Water on the Enthalpy of Solution of Cyclic Ethers in these Mixtures at <Emphasis Type="Italic">T</Emphasis> = 298.15 K

The enthalpies of solution of the cyclic ethers 1,4-dioxane, 12-crown-4 and 18-crown-6 in mixtures of ethanol and water have been measured within the whole mole fraction range at T = 298.15 K. The enthalpy of solvation has been calculated. In pure ethanol and pure water, the solvation enthalpy of the investigated cyclic ethers depends linearity on the number of –CH₂CH₂– groups in the cyclic ether molecules. Based on the analysis of the preferential solvation model proposed by Waghorne, it can be concluded that the 1,4-dioxane, 15C5 and 18C6 molecules are preferentially solvated by water molecules in the range of low water content in these mixtures. The effect of base–acid properties of ethanol–water mixtures on the enthalpy of solution of cyclic ethers in these mixtures has been analyzed. The enthalpy of solution of cyclic ethers correlates with the acidic properties of ethanol–water mixtures in the range of high and medium water content. The results presented are compared with analogous data obtained for the methanol–water and propan-1-ol–water mixtures.     

Studies on the uptake of different arsenic forms and the influence of sample pretreatment on arsenic speciation in White mustard (Sinapis alba)

The concentration and speciation of arsenic incorporated by plants grown in the presence of different arsenic compounds was compared, and the influence of plant sample pretreatment and extraction procedures on the recovery and reliability of speciation analyses was studied. It was concluded that sample pretreatment greatly affected the extraction efficiency, but did not change arsenic speciation. The most suitable extraction procedure involved dried plant material without the use of liquid nitrogen. To assess the ability of White mustard to uptake arsenic in different forms, samples were cultivated in nutrient solutions containing either As(III), As(V), monomethylarsonic acid (MMA) or dimethylarsinic acid (DMA). The translocation factor was the highest (0.70) when DMA was added to the nutrient solution, however the overall As concentration in plant tissues was the lowest in this case. Only inorganic As was found in plant tissues when either As(III) or As(V) was added to the nutrient solution. When organic arsenic was present in the nutrient medium, however, it was partially transformed by the plants into inorganic forms.     

Chemiluminescence of the Reaction System Ce(IV) - Non-Steroidal Anti-Inflammatory Drugs Containing Europium(III) Ions and its Application to the Determination of Naproxen in Pharmaceutical Preparations and Urine

The chemiluminescence (CL) of oxidation of non-steroidal anti-inflammatory drugs (NSAIDs) by Ce(IV) ions, was recorded in the presence and absence europium(III) ions, in solution of pH ~ 4 of solution. Kinetic curves and CL emission spectra of the all studied systems were discussed. CL of measurable intensity was observed in the Ce(IV)–NP–Eu(III) reaction system only in acidic solutions. The CL spectrum rcegistered for this system shows emission bands, typical of Eu(III) ions, with maximum at λ ~ 600 nm. The chemiluminescent method, based on Eu(III) emission in reaction system of NP-Ce(IV)–Eu(III) in acid solution was therefore used for the determination of naproxen in mixture of non-steroidal anti-inflammatory drugs.     

Prediction of protein structure using a knowledge-based off-lattice united-residue force field and global optimization methods

A united-residue model of polypeptide chains developed in our laboratories with united side-chains and united peptide groups as interaction sites is presented. The model is designed to work in continuous space; hence efficient global-optimization methods can be applied. In this work, we adopted the distance-scaling method that is based on continuous deformation of the original rugged energy hypersurface to obtain a smoothed surface. The method has been applied successfully to predict the structures of simple motifs, such as the three-helix bundle structure of the 10-58 fragment of staphylococcal protein A in de novo folding simulations and more complicated motifs in inverse-folding simulations. Received: 24 April 1998 / Accepted: 4 August 1998 / Published online: 2 November 1998     

Primary electron localization in polar liquids: Dependence on matrix and temperature

Computer generated structures of several polar liquids (water at 298 and 373 K, 1-propanol and 2-propanol at 298 K) have been analyzed in order to identify the regions of attractive potential that can serve as pre-existing sites for primary localization of an excess electron. Properties of pre-existing electron traps have been described in terms of statistical distributions of their geometrical parameters, energy levels and persistence times. The obtained results can explain, at least qualitatively, differences in the localization process in the considered matrices.     

Antimicrobial Food Packaging with Biodegradable Polymers and Bacteriocins

Innovations in food and drink packaging result mainly from the needs and requirements of consumers, which are influenced by changing global trends. Antimicrobial and active packaging are at the forefront of current research and development for food packaging. One of the few natural polymers on the market with antimicrobial properties is biodegradable and biocompatible chitosan. It is formed as a result of chitin deacetylation. Due to these properties, the production of chitosan alone or a composite film based on chitosan is of great interest to scientists and industrialists from various fields. Chitosan films have the potential to be used as a packaging material to maintain the quality and microbiological safety of food. In addition, chitosan is widely used in antimicrobial films against a wide range of pathogenic and food spoilage microbes. Polylactic acid (PLA) is considered one of the most promising and environmentally friendly polymers due to its physical and chemical properties, including renewable, biodegradability, biocompatibility, and is considered safe (GRAS). There is great interest among scientists in the study of PLA as an alternative food packaging film with improved properties to increase its usability for food packaging applications. The aim of this review article is to draw attention to the existing possibilities of using various components in combination with chitosan, PLA, or bacteriocins to improve the properties of packaging in new food packaging technologies. Consequently, they can be a promising solution to improve the quality, delay the spoilage of packaged food, as well as increase the safety and shelf life of food.     

Planarity of heteroaryldithiocarbazic acid derivatives showing tuberculostatic activity. II. Crystal structures of 3‐[amino(pyrazin‐2‐yl)methylidene]‐2‐methylcarbazic acid esters

Four compounds showing moderate antituberculostatic activity have been studied to test the hypothesis that the planarity of the 2‐[amino(pyrazin‐2‐yl)methylidene]dithiocarbazate fragment is crucial for activity. N′‐Anilinopyrazine‐2‐carboximidamide, C₁₁H₁₁N₅, D1, and diethyl 2,2′‐[({[amino(pyrazin‐2‐yl)methylidene]hydrazinylidene}methylidene)bis(sulfanediyl)]diacetate, C₁₄H₁₉N₅O₄S₂, B1, maintain planarity due to conjugation and attractive intramolecular hydrogen‐bond contacts, while methyl 3‐[amino(pyrazin‐2‐yl)methylidene]‐2‐methyldithiocarbazate, C₈H₁₁N₅S₂, C1, and benzyl 3‐[amino(pyrazin‐2‐yl)methylidene]‐2‐methyldithiocarbazate, C₁₄H₁₅N₅S₂, C2, are not planar, due to methylation at one of the N atoms of the central N—N bond. The resulting twists of the two molecular halves (parts) of C1 and C2 are indicated by torsion angles of 116.5 (2) and −135.9 (2)°, respectively, compared with values of about 180° in the crystal structures of nonsubstituted compounds. As the methylated derivatives show similar activity against Mycobacterium tuberculosis to that of the nonsubstituted derivatives, maintaining planarity does not seem to be a prerequisite for activity.     

Microwave-assisted Cannizzaro reaction—Optimisation of reaction conditions

The microwave-assisted Cannizzaro reaction was studied in order to develop fully reproducible synthetic protocols for transformation of aldehydes to carboxylic acid and alcohols. Optimised were the following process parameters: power, temperature, and time. Aromatic, heteroaromatic and aliphatic aldehydes were used in the studies. It was found that furfural, thiophene-2-carbaldehyde, pyridinecarboxaldehyde and aromatic aldehydes react under mild conditions, while 1-methyl-pyrrole-2-carboxaldehyde derivatives and aliphatic aldehydes require more drastic reaction conditions and a longer exposure time to microwave radiation.