Sonochemistry: a systematic study of the maleate-fumarate isomerization in the presence of alkyl bromides

A systematic and quantitative study of diethyl maleate-diethyl fumarate induced isomerization has been performed in CCl₄ using different ‘inert’ gases, different initiators and various experimental conditions. It was shown how changes in the experimental conditions, generally considered as minor in thermochemistry, can have dramatic effects in sonochemistry (for example, the ‘marble effect’).     

Nucleophile-assisted racemisations of halosilanes — kinetic studies

Kinetic studies have been carried out on the nucleophile-induced racemisation of PhMeCHSiMe₂X, 2, X = triflate, Br or Cl. Thirteen nucleophiles were studied. The results are interpreted in terms of two competing mechanisms for racemisation: (a) nucleophile attack on a silane-nucleophile complex formed by displacement of the halide by the nucleophile, and (b) halide-halosilane exchange, with inversion of configuration. Solvent effects were examined, and kinetic orders in the nucleophile and in one case for the halosilane were determined. The order in added nucleophile varied between one and two, with strong nucleophiles in polar media. Anomalously high orders in nucleophile were observed in non-polar media and are ascribed to aggregation of the nucleophile. A kinetic analysis of the competing mechanisms was attempted, and was consistent with the experimental findings. In this particular series of reactions involving compounds with good leaving groups and relatively powerful nucleophiles there was no evidence for intermediates involving extracoordinate silicon.     

Simultaneous determination of vanadium and lead in unsegmented flow systems of variable flow rate

Summary A completely continuous flow system and one with injection of the sample based on variable flow rates were used to develop a simultaneous determination for vanadium(V) and lead(II). Each metal ion is complexed with 4-(2-pyridyl-azo) resorcinol at a different pH which was established in the flow system by changing the flow rate. The features of the proposed determinative methods are discussed and compared with those of others reported in the literature.     

Poly(hydroxybutyrate) and epichlorohydrin elastomers blends: Phase behavior and morphology

This work studied blends of PHB with epichlorohydrin elastomers, the PEP homopolymer and its copolymer with ethylene oxide, ECO. PHB is a microbial polyester, which is accumulated intracellularly by a large number of microorganisms, presenting characteristics of biodegradability and biocompatibility. It presents a high degree of crystallinity, so is a quite brittle material, and may undergo degradation when is kept for a relatively short time at a temperature above its melting point, about 180 °C. PEP and ECO are linear and amorphous elastomers, exhibit miscibility with many aliphatic polyesters and these elastomers have been used in various branches of technology, such as the automotive industry. The proposed systems combine a polymer with high crystallinity and biodegradability, PHB, with amorphous epichlorohydrin elastomers. Blends were prepared by casting from chloroform solution at different compositions (0, 20, 40, 50, 60, 80 and 100 wt% of PHB). The phase behavior of PHB/PEP and PHB/ECO blends were studied by differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA) and the morphology of the crystalline phase of PHB had been examined by optical microscopy. Blends of PHB/PEP and PHB/ECO have been described in literature as miscible. However, our results from the DSC and DMA show that PHB/PEP and PHB/ECO blends are immiscible. This behavior should be related to the molecular weight of polymers used in the present work, which is higher than the molecular weight of polymers used in the previous works. The crystallization kinetics of PHB is strongly influenced by the presence of the elastomeric phase. The degree of crystallinity of PHB/PEP blends decreases with an increase in the PEP content. PHB/ECO blends present degrees of crystallinity that can be considered nearly independent of the ECO content. Differences in the morphology of the crystalline phase were also observed, and these are attributed to the presence of elastomeric phase in the intraspherulitic zone.     

Thermally reversible hydrogels via intramolecular folding and consequent self-assembly of a de novo designed peptide

A small de novo designed peptide (MAX3) is described that exhibits complete thermoreversible self-assembly into a hydrogel network. Importantly, a prerequisite to hydrogelation is that the peptide must first fold into a conformation conducive to self-assembly. At ambient temperature, MAX3 is unfolded, resulting in a low viscosity aqueous solution. On increasing the temperature, the peptide undergoes a unimolecular folding event, affording an amphiphilic beta-hairpin that consequently self-assembles into a hydrogel network. Increasing the temperature serves to dehydrate the nonpolar residues of the unfolded peptide and trigger folding via hydrophobic collapse. Cooling the resultant hydrogel results in beta-hairpin unfolding and consequent complete dissolution of the hydrogel. The temperature at which folding and consequent self-assembly into a rigid hydrogel occur can be tuned by altering the hydrophobicity of the peptides.     

Susceptibly of Alphitobius diaperinus to Beauveria bassiana extracts

Alphitobius diaperinus is an important pathogen with worldwide distribution that causes severe economic loss of efficiency in broilers. This study evaluates the potential of organic extracts of two strains entomopathogenic fungus Beauveria bassiana (CG71 and UNI40) as a biocontrol agent on A. diaperinus and promotes the phytochemical investigation. The effective percentages of mortalities were 95.97% (UNI40 methanolic extract), 69.23, 64.64, and 50.39% (CG 71 methanolic, ethyl acetate and butanol extracts). However, there was a decrease in the lesser mealworms susceptibility in relation to the use of insecticides and extracts. The metabolites 5-hydroxymethyl-2-furanoic acid, dipicolinic acid and monomethyl dipicolinate were isolated from ethyl acetate extract, and β-adenosine of butanolic extract of B. bassiana CG 71. In addition, the cyclodepsipeptides were identified in methanolic extracts of the two strains. The insecticide activity results indicated that the B. bassiana extracts are an alternative to A. diaperinus control.

     

Simultaneous determination of food dyes by first derivative spectrophotometry with sorption onto polyurethane foam.

A simple spectrophotometric method is described for resolving binary mixtures of some food dyes: Amaranth, Brilliant Blue, Sunset Yellow and Tartrazine, using the first-derivative spectra with measurements at zero-crossing wavelengths. Analytical curves are linear up to 20 mg L(-1). Standard deviations of 1.30, 2.22, 1.93 and 0.81% were obtained for synthetic binary mixtures of 2 mg L(-1) of Amaranth, Brilliant Blue, Sunset Yellow and Tartrazine, respectively. Before the spectrophotometric measurements, the dyes were sorbed onto polyurethane foam and recovered in sodium dodecyl benzene sulfonate solution. Therefore, matrix complexity was eliminated and simple spectra were obtained. The method was very satisfactorily used for determining the colorants in synthetic mixtures, with recoveries in the 96 - 101% range. Detection limit values were dependent on the colorant combination investigated. Commercial products containing binary combinations of these dyes in different ratios (from 1:1 to 1:8) were analyzed. The results were compared with those obtained by HPLC; very similar values were found by the two methods.     

Design and application of basic amino acids displaying enhanced hydrophobicity

Three noncoding basic amino acids, mono-, di-, and trimethyldiaminopropionic acid (mmdap, dmdap, and tmdap), have been synthesized for use in protein design. Covalent modification of a diaminopropionic acid (dap) side chain with an increasing number of methyl moieties results in a family of residues displaying short basic side chains with varying degrees of enhanced hydrophobic character. These residues may be used to introduce charged/polar interactions into the confining hydrophobic interior or interfacial spaces of proteins. As a demonstration of their utility, the ability of these residues to promote interior salt bridge formation at the helix/helix interface of GCN4-p1, a dimeric two-stranded coiled coil, was assessed. Heterodimerization mediated by buried salt bridge formation between a GCN4-based peptide containing either mmdap, dmdap, or tmdap at position 16 and an analogous peptide containing aspartic acid at the same position was studied. Mmdap-derived heterodimers are 0.5 kcal/mol more stable than the corresponding dap-derived heterodimers. This result indicates that the addition of one methyl group to the dap side chain can stabilize the heterodimeric fold. The stabilization can most likely be attributed to a decrease in the desolvation penalty incurred upon folding as well as enhanced van der Waals contacts in the folded state. The addition of three methyl groups to the dap side chain results in heterodimers that are significantly less stable than the corresponding dap-derived heterodimers, suggesting that increased steric bulk is not well accommodated in the interior of this protein. Unexpectedly, the addition of two methyl groups leads to homotrimerization of the dmdap-peptide. The resulting trimer is relatively stable (DeltaG(37)( degrees )(C) degrees = 11.8 kcal/mol) and undergoes cooperative thermal unfolding. The GCN4-p1 system exemplifies how small incremental changes in size and hydrophobicity can alter the folding preferences of a protein. Generally, this versatile suite of residues can be utilized in any protein and offer new options to the protein chemist.