Thin-Layer Chromatographic Determination of the pKa Values of Organic Acids and Bases

JPC – Journal of Planar Chromatography – Modern TLC -     

Spectroelectrochemical properties of poly(4,4'-dialkyl-2,2'-bithiophenes) and poly( 3-alkylthiophenes): A comparative study

The influence of alkyl side chain distribution patterns on the Spectroelectrochemical properties of poly(alkylthiophenes) has been studied. Two types of compounds were studied as representing different coupling patterns:(i) poly(4,4'-dialkyl-2,2'-bithiophenes), which represent head-to-head and tail-to-tail coupled poly(al-kylthiophene) chains; and(ii) poly(3-alkylthiophenes), which represent head-to-tail coupled poly(alkylthiophene) chains.Both types of polymers can be obtained chemically or electrochemically by the constant potential method or the potential scanning method. Poly(4,4'-dialkyl-2,2'-bithiophenes) differ significantly from poly(3-alkylthiophenes) in their voltammetric and spectroelectrocheinical properties. The oxidative doping of these compounds occurs in a very narrow potential range (35 mV as determined by static Spectroelectrochemical studies) and is significantly retarded in dynamic measurements. Analysis of the changes in absorption in the 500 nm spectral region and the observation of an induction period in the oxidation at constant potential seem to indicate that the oxidative doping is preceded by structural changes which facilitate the oxidation.Although in the neutral state poly(4,4'-dialkyl-2,2'-bithiophenes) absorb at 390 nm, i.e. their π→π^* transition is blue-shifted as compared to poly(3-alkylthiophenes), the polaronic states included by doping are located in the same spectral region for both families of polymers. This observation is consistent with the postulate of significant changes induced by doping in poly(4,4'-dialkyl-2,2'-bithiophenes). The more twisted head-to-head and tail-to-tail coupled poly(alkylthiophene) chains must adopt a more planar structure, similar to poly(3-alkylthiophenes), in order to create polaronic states and accommodate the charge-compensating anions.     

Thwarts in transversal designs

A subset of points in a transversal design is athwart if each block in the design has one of a small number of intersection sizes with the subset. Applications to the construction of mutually orthogonal latin squares are given. One particular case involves inequalities for the minimum number of distinct symbols appearing in an × subarray of an×n latin square. Using thwarts, new transversal designs are determined for orders 408, 560, 600, 792, 856, 1046, 1059, 1368, 2164, 2328, 2424, 3288, 3448, 3960, 3992, 3994, 4025, 4056, 4824, 5496, 6264, 7768, 7800, 8096, and 9336.     

IA,database of known ligands of aminoacyl-tRNA synthetases

The IA database contains 240 structures of known inhibitors of aminoacyl-tRNA synthetases. Structures can be downloaded in different file formats (mol, sdf, smile, png). The search engine offers possibility of searching for the ligands with a given functional group. Additionally, one can search for ligands that act on selected synthetases and from particular references. The data include information which synthetase a given ligand inhibits together with the inhibition constant (IC₅₀) if known. Database is freely available at     

Photochemical and electrochemical Z–E isomerization of 1,4-dialkoxy-2,5-bis[2-(thien-2-yl)ethenyl]benzene stereoisomers

The group of 1,4-dialkoxy-2,5-bis[2-(thien-2-yl)ethenyl]benzene stereoisomers was synthesized in which methoxy- and ethoxy-groups were used as alkoxy-substituents. These isomers were characterized as solution species both electrochemically and spectroscopically. As expected, these compounds, having a stilben like structure, are the subject of photoisomerization, which is described and discussed. It is demonstrated how electrochemical process may cause isomerization of the double CC bonds in that group of compounds. An attempt of using electrochemical methods to monitor the process of photoisomerization of these compounds is presented. Mechanism of the oxidatively induced electrochemical isomerization has been proposed and discussed. The electrochemical isomerization mechanism is verified by digital simulation, which allowed estimating basic kinetic parameters of the processes.     

Five mutually orthogonal Latin squares of order 35

Let N(n) denote the maximum number of mutually orthogonal Latin squares of order n. It is shown that N(35) ≥ 5. © 1996 John Wiley & Sons, Inc.     

Twisted $$\Gamma \times \mathbb T^ n$$Γ×T n-equivaria nt degree with n-parameters: computatio nal formulae a nd applicatio ns

We introduce the generalized twisted \(\Gamma \times \mathbb {T}^{n}\)-equivariant degree with n free parameters, where \(\Gamma \) is a nonabelian finite group, present a computational formulae based on the reduction to \(\mathbb {T}^{n}\)-equivariant maps and reduction of parameters techniques. This twisted equivariant degree can be effectively applied to study Hopf bifurcation with symmetries from relative equilibria. We give an example of such bifurcation in a system of ODEs.     

Xenon-sensitized radiolysis of hydrogen sulphide in the presence of HBr,N2O,CCl4, and SF6

The γ-radiolysis of hydrogen sulphide in the excess of xenon : as studied as a function of added HBr, N₂O, CCl₄ and SF₆. Addition of HBr results in the considerable increase of H₂ yield while addition of excess of N₂O, CGl₄ and SF₆ reduces G(H ₂ ) to almost zero. When N₂O is present, the N₂ yield also varies with the H₂S/N₂O ratio, being uninfluenced by the addition of small amounts of SF₆ Consideration of possible ionic and nonionic reactions leads to the conclusion that hot hydrogen atoms play a major role in the formation both of hydrogen and nitrogen. The mechanism of electron attachment and ion recombination processes is proposed which is consistent with all the obtained data.     

Efficiency of superoxide anions in the inactivation of selected dehydrogenases

The most ubiquitous of the primary reactive oxygen species, formed in all aerobes, is the superoxide free radical. It is believed that the superoxide anion radical shows low reactivity and in oxidative stress it is regarded mainly as an initiator of more reactive species such as OH and ONOO^‐.In this paper, the effectiveness of inactivation of selected enzymes by radiation-generated superoxide radicals in comparison with the effectiveness of the other products of water radiolysis is examined. We investigate three enzymes: glyceraldehyde-3-phosphate dehydrogenase (GAPDH), alcohol dehydrogenase (ADH) and lactate dehydrogenase (LDH).We show that the direct contribution of the superoxide anion radical to GAPDH and ADH inactivation is significant. The effectiveness of the superoxide anion in the inactivation of GAPDH and ADG was only 2.4 and 2.8 times smaller, respectively, in comparison with hydroxyl radical. LDH was practically not inactivated by the superoxide anion.Despite the fact that the studied dehydrogenases belong to the same class of enzymes (oxidoreductases), all have a similar molecular weight and are tetramers, their susceptibility to free-radical damage varies. The differences in the radiosensitivity of the enzymes are not determined by the basic structural parameters analyzed. A significant role in inactivation susceptibility is played by the type of amino acid residues and their localization within enzyme molecules.