53rd Meeting of the International Society of Electrochemistry

Russian Journal of Electrochemistry -     

Magnetic-Field-Dependent Electronic Relaxation of Gd3+ in Aqueous Solutions of the Complexes [Gd(H2O)8]3+, [Gd(propane-1,3-diamine-N,N,N′,N′-tetraacetate)(H2O)2]−, and [Gd(N,N′-bis[(N-methylcarbamoyl)methyl]-3-azapentane-1,5-diamine-3,N,N′-triacetate)(H2O)] of interest in magnetic-resonance imaging

EPR Spectra have been measured for aqueous solutions of a series of Gd³⁺ complexes at variable temperature and a range of magnetic fields; S-band (0.14 T), X-band (0.34 T), Q-band (1.2 T), and 2-mm-band (5.0 T). The major contribution to the observed line widths is magnetic-field-dependent and is interpreted as being due to the modulation of the zero-field splitting produced by distortion of the complexes from perfect symmetry. The transverse and longitudinal relaxation matrices for an ⁸S ion with such an interaction have been calculated using Redfield theory with vector-coupling methods, and diagonalised numerically to obtain relaxation rates and intensities for the degenerate transitions which contribute to the multiplet. The observed line width, which is inversely proportional to the magnetic field at low temperatures, is best described by the intensity-weighted mean transverse relaxation time for the four transitions with non-zero intensity. A least-squares fit of the data yields the square of the zero-field splitting tensor, Δ², and a correlation time, τ_v, with activation energy, E_v. The physical significance of these parameters and the extent of validity of the theoretical approach are considered. The parameters are used to predict the magnetic-field dependence of the longitudinal and transverse electronic relaxation times, which are discussed in the context of their relevance to ¹H-NMR relaxivity.     

Electrochemical Fluorosulfation of Fluoroalkenes: Factors Determining the Process Selectivity

A comparison of results of indirect and direct electrochemical fluorosulfation of nonsaturated fluorocarbons shows that the main or the only product in potentiostatic conditions is vicinal fluorosulfates. Owing to a high rate of the electrochemical step, which exceeds the rate of desorption of intermediate radicals from the electrode surface into solution, it is possible to improve the process selectivity by increasing the contribution of the ionic reaction route.     

Electrochemical perfluoroalkoxylation of aromatic compounds

New approaches to the electrochemical synthesis of aryl perfluoroalkyl ethers based on a) the electrooxidation of perfluorocarboxylic acid on a consumable PbO₂ anode and b) the simultaneous anodic oxidation of the “aromatic compound-alkaline perfluoroalkoholate” system on a platinum electrode were proposed.     

Behavior of protein-like N-vinylcaprolactam and N-vinylimidazole copolymers in aqueous solutions

The free-radical copolymerization of N-vinylcaprolactam and N-vinylimidazole (at an initial comonomer ratio of 85: 15, mol/mol) initiated by a persulfate-tertiary amine redox system in 10% aqueous DMSO at 25 and 65°C (at temperatures below and above the temperature of phase separation in the reaction system, respectively) yielded macromolecular products that were subsequently separated into thermally precipitating and nonprecipitating fractions. Investigations of these fractions by capillary viscometry, static and dynamic laser light scattering, and high-sensitivity DSC showed that macromolecules of both types of copolymers are strongly associated in aqueous solutions. Upon heating of solutions of thermally nonprecipitating fractions, additional aggregation takes place and this phenomenon is accompanied by a decrease in the size of particles without loss in their solubility until at least 70°C is reached. As for the set of properties exhibited in aqueous solutions, the thermally nonprecipitating fraction of the copolymer synthesized at 65°C may be assigned to protein-like macromolecules.     

Untersuchung der Wechselwirkung von Eiweißen mit Polysacchariden im Wassermedium

Zusammenfassung Die komplexe Koazervation im SystemG-A-H₂O kann als zweistufiger Prozeß aufgefaßt werden. Das erste Stadium ist die Bildung des elektrisch neutralen Komplexes zwischen G und A unter Einfluß der elektrostatischen Anziehungskräfte. Die Stöchiometrie der gewonnenen Verbindung wird durch die Äquivalenz der Makroionen bestimmt.Nur elektrisch neutrale Komplexe können die neue Phase, d. h. das komplexe Koazervat bilden. Deshalb muß die Zusammensetzung der koazervaten Phase von p_H abhängen und bei Veränderung des Verhältnisses der Polymere im Gemisch unveränderlich bleiben.Bei einem Verhältnis der Polymere, kleiner als das stöchiometrische, ist die Bildung geladener Komplexe möglich.Den Prozeß der komplexen Koazervation kann man mittels der Methode der Trübungsmessung bei C_s<6·10^{–3 g}/100 g untersuchen. Hierbei ist die optische Dichte proportional der Menge des komplexen Koazervats oder des elektrisch neutralen Komplexes im System.     

Thermodynamic study of N-trifluoroacetyl-O-alkyl nipecotic acid ester enantiomers on diluted permethylated beta-cyclodextrin stationary phase

Thermodynamic studies were performed on 12 pairs of N-trifluoroacetyl-O-alkyl nipecotic acid ester enantiomers on diluted permethylated beta-cyclodextrin stationary phase (CP Chirasil-Dex CB). The influence of ester alkyl group structure on interaction with permethylated beta-cyclodextrin (Me-CD) and enantioselectivity was studied. The types of alkyl groups studied included n-alkyl (C1-C5) and groups containing branching at differing locations relative to the chiral center of the molecule. The results show that for a given molecular weight, the n-alkyl esters have stronger interactions with Me-CD than esters containing branched alkyl groups. However, although having weaker interactions with Me-CD, esters containing alpha-branched alkyl groups exhibit higher enantioselectivity than the corresponding n-alkyl or beta-branched isobutyl esters. From the retention data, thermodynamic parameters were estimated using the retention increment method and enthalpy-entropy compensation plots (ln R' versus deltaH) were constructed. The results suggest that ester enantiomers with branching at the alpha-carbon of the ester alkyl group have additional and/or different types of enantioselective interactions with Me-CD than the C1-C5 n-alkyl esters or beta-branched isobutyl ester. In order to obtain a qualitative sense of the interaction with Me-CD, structures of the diastereomeric complexes formed between Me-CD and some of the ester enantiomers were modeled using simulated annealing molecular dynamics.     

Thermotropic gelation of ovalbumin 1. Viscoelastic properties of gels as a function of heating conditions and protein concentration at various pH values

A study has been undertaken of stress relaxation in ovalbumin thermotropic gels with a concentration of 8–20%, depending on time and temperature of heating (respectively, 20–60 min, 70°–110°C), at pH 2.5–10.0. In all instances, the dependence of the initial gel elasticity modulus on heating has a single maximum. Gelation conditions corresponding to this maximum are considered optimal. Optimal gelation time is 30 min, regardless of pH. On the other hand, the optimal heating temperature depends on pH. To the right and left of the isoelectric point of protein (2.5pH<4.0 and 5.5G) of gels on heating conditions, pH and protein concentration (X ₁,X ₂,X ₃,X ₄), as well as on time of relaxation (t) may be generally described asG(X ₁,X ₁,X ₁,X ₁,t)=G _e(X ₁,X ₂,X ₃,X ₄)f(t), whereG _e is the equilibrium value of the elasticity modulus, and f(t) the relaxation function. Thus, a change in the parameters only affects the value of the equilibrium elasticity modulus, and exerts no effect on the relaxation time spectrum. For this reason, all the relaxation curves obtained may be transformed into two normalized relaxation functions:f(t)=f(t)/f(1)=G(X ₁,X ₂,X ₃,X ₄,t)/G(X ₁,X ₂,X ₃,X ₄, 1)Each of these normalized functions corresponds to one of the homologous groups. Rheological similarity of gels in each homologous group evidently points to their structural similarity. Invariance of the gel relaxationproperties with regard to protein concentration, leads to a concentration dependence of the equilibrium modulus at various pH values. These dependences are curvilinear on a double logarithmic scale. The slope of the curve exceeds 2 in the entire concentration interval studied. In other words, the dependences obtained cannot be described by the usual law of squares. On the other hand, they adequately match Hermans theoretical relation for a network formed by random association of identical polyfunctional particles without cyclization. This simple model evidently gives a true picture of the major regularities of thermotropic gelation for ovalbumin. An agreement between this theory and experiment was achieved for a protein concentration ofC ^*=6.0±1.0% at the gel point regardless of pH. Invariance of gelpoint position with regards to pH demands further confirmation.List of symbols T _h,t _h heating temperature and time - T _h ^* ,t _h ^* optimal heating temperature and time - C protein concentration - C ^* protein concentration in gel-point - G relaxation modulus - G _e equilibrium modulus - f(t) relaxation function - t time of relaxation - f(t) normalized relaxation function - fT _A (t), f _B (t) normalized relaxation functions of groups A and B - G ₁ T _h,t _h-reduced modulus - G ₂ T _h,t _h, pH-reduced modulus - G ₃ C-reduced modulus - b ₁ T _h, t_h reduction parameter of modulus - b ₂ pH reduction parameter of modulus - b ₃ C reduction parameter of modulus - W_g gel-fraction     

Micellar enhanced cyanide ion determination in samples from synthetic organic processes

A method was developed for the recovery and determination of cyanide ion in organic sample matrices. To facilitate the solubilization of cyanide ions, cetyltrimethylammonium bromide (CTAB) was added at concentrations above the critical micelle concentration. Sample cyanation reaction products consisted of solvent mixtures of a hydroxynitrile in DMF-toluene or DMF-isopropylacetate (IPAC). Spectrophotometric determination of cyanide ion at 578 nm by the pyridine-barbituric acid method was automated by flow injection analysis. Recovery of cyanide ion from spiked samples was 93.2% in DMF-IPAC solvent matrix and 93.9% in DMF-toluene. Low alkali concentration was observed to favor solubilization of cyanide ion in the micellar solution.     

A vector-sum theorem in two-dimensional space

Given a finite setX of vectors from the unit ball of the max norm in the twodimensional space whose sum is zero, it is always possible to writeX = {x₁, , x_n} in such a way that the first coordinates of each partial sum lie in [–1, 1] and the second coordinates lie in [–C, C] whereC is a universal constant.