Voltamperometric determination of kinetin with a carbon paste modified electrode

The electrochemical behaviour of kinetin (6-furfurylaminopurine) on a carbon paste modified with OV-17 silicone electrode, is studied. The determination of kinetin is possible working in square wave voltammetric techniques, reaching limits of determination of 38.7 ng ml⁻¹. The proposed method was successfully applied to determine the cytokinin in extracts of apples (previously spiked with kinetin) and the obtained results were in accordance with the results obtained with HPLC-UV.     

Oxidation of formic acid and carbon monoxide on gold electrodes studied by surface-enhanced Raman spectroscopy and DFT.

The oxidation of formic acid and carbon monoxide was studied at a gold electrode by a combination of electrochemistry, in situ surface-enhanced Raman spectroscopy (SERS), differential electrochemical mass spectrometry, and first-principles DFT calculations. Comparison of the SERS results and the (field-dependent) DFT calculations strongly suggests that the relevant surface-bonded intermediate during oxidation of formic acid on gold is formate HCOO- ad*. Formate reacts to form carbon dioxide via two pathways: at low potentials, with a nearby water to produce carbon dioxide and a hydronium ion; at higher potentials, with surface-bonded hydroxyl (or oxide) to give carbon dioxide and water. In the former pathway, the rate-determining step is probably related to the reaction of surface-bonded formate with water, as measurements of the reaction order imply a surface almost completely saturated with adsorbate. The potential dependence of the rate of the low-potential pathway is presumably governed by the potential dependence of formate coverage. There is no evidence for CO formation on gold during oxidation of formic acid. The oxidation of carbon monoxide must involve the carboxyhydroxyl intermediate, but SERS measurements do not reveal this intermediate during CO oxidation, most likely because of its low surface coverage, as it is formed after the rate-determining step. Based on inconclusive spectroscopic evidence for the formation of surface-bonded OH at potentials substantially below the surface oxidation region, the question whether surface-bonded carbon monoxide reacts with surface hydroxyl or with water to form carboxyhydroxyl and carbon dioxide remains open. The SERS measurements show the existence of both atop and bridge-bonded CO on gold from two distinguishable low-frequency modes that agree very well with DFT calculations.     

Base-induced ring cleavage of 4-functionalized-3-unsubstituted isoxazoles. Synthesis of 5-aminoazoles and 4-cyanoazoles

The base-induced ring cleavage of 4-nitro-(Ia), 4-ethoxycarbonyl- (Ib) and 4-acetyl-5-methylisoxazole (Ic) and the conversion of the resulting β-cyanoenolates and β-enaminonitriles into 5-aminoazoles and 4-cyanoazoles was studied.     

Derivatives of 2-thiohydantoin as spectrophotometric analytical reagents-II Condensation at C-5 with aromatic non-pyridine aldehydes

The synthesis, characteristics, properties and reactions with metallic ions of six aromatic derivatives of 2-thiohydantoin have been studied. The reagents exhibit two pK values in aqueous solution, and with Pd(II), Au(III), Ag(I) and Hg(I) and (II), especially with the first, form stable complexes with molar absorptivities adequate for spectrophotometric measurements.     

Reaction of tetrahydrobenz[a]acridinones with hydroxylamine hydrochloride. VII

Oximation of ortho-substituted phenylbenz[a]acridinones using hydroxylamine hydrochloride, sodium hydroxide and ethanol as the solvent gave always the benzoquinacridine N-oxide 2 . Oximation of para-substituted phenylbenz[a]acridinones, however, gave only the corresponding oximes. The structure of all products was corroborated by ir, ¹H and ¹³C-nmr and mass spectral data. Theoretical calculations support the experimental findings.     

Acid Pullulanase from Bacillus polymyxa MIR-23

Within the frame of a screening program aimed at the isolation of amylolytic sporeformers, one strain with high amylolytic activity designated MIR-23 was selected. The microbial characterization was carried out by morphological and biochemical tests and, by means of statistical treatment, was identified asBacillus polymyxa. The organism could grow in acidic conditions (pH 5.0) on a starch medium and produce α-amylase, pullulanase, and α-glucosidase. Batch cultures showed the highest enzyme activities in the stationary phase. Pullulanase activity exhibited an optimal temperature of 52–57°C at pH 4.5–5.5. These properties would allow its use in the saccharification processes in the starch industries.     

Considerations on the dependence of gas–liquid chromatographic retention of n-alkanes with the carbon number

The contributions to the free energy change involved in the chromatographic retention of n-alkanes, which are functions of the solute chain length, are discussed with the aid of recent advances in chain statistics. In the context of a simplified theoretical analysis, a discussion is promoted on the molecular nature of the observed slight deviations from linearity of ln t_R′ vs. n. A non-linear function ln t_R′(n) is derived from the application of the generalized Van der Waals fluid partition function to the liquid stationary phase. The excluded volume, particularly its dependence on n, is analyzed through the theory of Flory for dilute solutions of chain molecules.     

Minimal boundaries enclosing a given volume

We prove some facts concerning surfaces of minimal area bounding regions of prescribed volume in ⁿ. The main result we prove is that the mean curvature of such a surface is constant, if possibly a discontinuous function of the enclosed volume. The boundary behaviour of the solutions is also discussed.     

Evaluation of Ptitsyn-Eizner λ parameter in dilute solutions of poly(N-vinylcarbazole) and its variation in conformational transitions of poly(methyl methacrylate)

The Ptitsyn-Eizner λ flexibility parameter for poly(N-vinylcarbozole) solutions under theta conditions has been found to be 6.31, demonstrating the rigidity imparted to the polymer chain by the carbazole group. A study has been made of the variation of this parameter with solvent and temperature. A study has been made of the variation of λ in a conformational transition of poly(methyl methacrylate).     

Stabilization energies of the hydrogen-bonded and stacked structures of nucleic acid base pairs in the crystal geometries of CG, AT, and AC DNA steps and in the NMR geometry of the 5'-d(GCGAAGC)-3' hairpin: Complete basis set calculations at the MP2 and CCSD(T) levels

Stabilization energies of the H-bonded and stacked structures of a DNA base pair were studied in the crystal structures of adenine-thymine, cytosine-guanine, and adenine-cytosine steps as well as in the 5'-d(GCGAAGC)-3' hairpin (utilizing the NMR geometry). Stabilization energies were determined as the sum of the complete basis set (CBS) limit of MP2 stabilization energies and the Delta E(CCSD(T)) - Delta E(MP2) correction term evaluated with the 6-31G*(0.25) basis set. The CBS limit was determined by a two-point extrapolation using the aug-cc-pVXZ basis sets for X = D and T. While the H-bonding energies are comparable to those of base pairs in a crystal and a vacuum, the stacking energies are considerably smaller in a crystal. Despite this, the stacking is still important and accounts for a significant part of the overall stabilization. It contributes equally to the stability of DNA as does H-bonding for AT-rich DNAs, while in the case of GC-rich DNAs it forms about one-third of the total stabilization. Interstrand stacking reaches surprisingly large values, well comparable to the intrastrand ones, and thus contributes significantly to the overall stabilization. The hairpin structure is characterized by significant stacking, and both guanine...cytosine pairs possess stacking energies larger than 11.5 kcal/mol. A high portion of stabilization in the studied hairpin comes from stacking (similar to that found for AT-rich DNAs) despite the fact that it contains two GC Watson-Crick pairs having very large H-bonding stabilization. The DFT/B3LYP/6-31G** method yields satisfactory values of interaction energies for H-bonded structures, while it fails completely for stacking.