Properties of poly(vinyl alcohol)-graft-polyacrylamide copolymers depending on the graft length. 3. Benzene solubilization by solutions of the copolymer

The peculiarities of poly(vinyl alcohol)-graft-polyacrylamide copolymers (PVA-g-PAA), which are characterized by the equal average number (N=9), but various molecular weight (or length) of graft chains, in comparison with individual PAA and PVA, were investigated in aqueous medium. Sharp rise in benzene solubilization in PVA-g-PAA solutions at M_PAA higher than 4.3·10⁵ has been established. It was shown that such effect is stipulated by the destruction of intramolecular polymer-polymer complex in the copolymer and increasing the benzene binding to separate PVA-g-PAA groups by means of hydrogen bonds. The changes in the PVA-g-PAA solubilizing ability as the function of temperature were also investigated. The obtained results are discussed from the point of view of conformational transitions of intramolecular polymer-polymer complexes (intraPPC), which exist in copolymers, in dependence on the length of graft chains.     

Intramolecular cyclization of ω‐haloalkylsubstituted thiophosphorylacetonitriles: Synthesis and stereochemistry of 3‐cyano‐2‐oxo‐1,2‐thiaphosphacyclanes

Intramolecular S‐alkylation in a series of ω‐haloalkyl‐substituted thiophosphorylacetonitriles 5–7 presents an effective synthetic route to the hitherto unknown 3‐cyano‐2‐oxo‐1,2‐thiaphospholanes 14 and thiaphosphinanes 15 . The compounds were obtained as a mixture of cis‐ and trans‐isomers that were resolved to individual stereoisomers in most cases. For some of them, X‐ray diffraction analysis has been performed. It was shown that ³¹P NMR spectroscopy can be used to assign the stereochemistry of 3‐cyano‐2‐oxo‐1,2‐thiaphosphacyclanes. © 2002 John Wiley & Sons, Inc. Heteroatom Chem 13:1–21, 2002; DOI 10.1002/hc.1101     

Isotope effects in α-PdH(D) as an instrument for diagnosing bulk defects

Protium and deuterium sorption in the α-phase region is studied for highly defective palladium electrodeposits fabricated under controllable potentiostatic modes, particularly in the region of concentrated hydride formation. An anomalously high hydrogen content is observed for these samples in both α- and β-hydrides. On the basis of coulometry in the course of anodic hydrogen extraction, the non-linear sorption isotherms are plotted and their specific features are considered under an assumption of simultaneous equilibrium sorption of hydrogen by several types of lattice positions with a certain degree of defectiveness. Less defective palladium samples deposited in the absence of bulk hydridization are studied for comparison. The approach is proposed to estimate the fraction of defective regions. The procedure of analyzing the low-pressure limit is used for the first time for determining the specific values of the isotope effect, and also H and D Sievert's constants. The isotope effect is demonstrated to be extremely sensitive to the type of defectiveness. Electronic Publication     

Synthesis of difurazanyl ethers from 4,4′‐dinitroazoxyfurazan

Nonselective attacks at the carbon bonded to a nitro group and carbon bonded to the N(O) atom of the azoxy group were observed in the reactions of 4,4′‐dinitroazoxyfurazan with bases and nucleophiles. A mechanism is presented to account for both of the pathways to products. A series of new difurazanyl ether derivatives was synthesized. © 2000 John Wiley & Sons, Inc. Heteroatom Chem 11:48–56, 2000     

Octakis(ɛ‐caprolactam‐κO)erbium(III) hexaisothiocyanatochromate(III)

The title compound, [Er(C₆H₁₁NO)₈][Cr(NCS)₆], is a new structure type for [Ln(ɛ‐caprolactam)₈][Cr(NCS)₆], where Ln is a lanthanide. There are two independent cations and two independent anions in the asymmetric unit. The Er atom is in a distorted square‐antiprismatic environment of eight O atoms of the organic ligands; Er—O bond distances are in the range 2.29–2.44 Å. The coordination environment of the Cr atom is a slightly distorted octahedron; Cr—N bond distances vary from 1.99 to 2.01 Å. The mutual packing of the cations and the anions follows a distorted NaCl motif. One cation has the Er atom on a twofold axis and one of the anions has the Cr atom on a twofold axis.     

Phase Transitions in Dynamical Random Graphs

We study a large-time limit of a Markov process whose states are finite graphs. The number of the vertices is described by a supercritical branching process, and the dynamics of edges is determined by the rates of appending and deleting. We find a phase transition in our model similar to the one in the random graph model G _n,p . We derive a formula for the line of critical parameters which separates two different phases: one is where the size of the largest component is proportional to the size of the entire graph, and another one, where the size of the largest component is at most logarithmic with respect to the size of the entire graph. In the supercritical phase we find the asymptotics for the size of the largest component.     

Using enzymes isolated from diverse sources to determine metal ion cofactors

Oxidoreductases and hydrolases isolated from different sources (horseradish and peanut peroxidases, alcohol dehydrogenases from bakers yeast and horse liver, and alkaline phosphatases from Escherichia coli, chicken and seal intestine) were used to determine their metal ion cofactors: Fe(III), Zn(II) and Mg(II), respectively. Studying the effects of the metal ion cofactors on the catalytic activity of the enzymes of different origin showed that the extent of their inhibition, activation, or reactivation of their apoenzymes depended on the structure and accessibility of the enzyme active site, which varies among the biocatalysts isolated from different sources. The developed procedures are based on the inhibiting (Zn(II)) or activating (Mg(II)) effects of the metal ions on the catalytic activity of the enzymes, or on reactivating effects (Fe(III) and Zn(II)) on the apoenzymes. The procedures are characterized by high sensitivity and selectivity; the detection limits of Fe(III) using horseradish peroxidase, Zn(II) using alcohol dehydrogenase from bakers yeast, alkaline phosphatase from seal intestine and its apoenzyme, and Mg(II) using alkaline phosphatase from chicken intestine equal 10 ng L^–1, 20 ng L^–1, 3 g L^–1, 8 g L^–1 and 0.2 g L^–1, respectively.Presented at the 8th Symposium Kinetics in Analytical Chemistry, Rome, Italy, 5–8 July 2004.     

Complexation of lappaconitine with glycyrrhizic acid: stability and reactivity studies

NMR and UV-vis spectroscopy have been used to study the complexation of antiarrhythmic alkaloid lappaconitine with an efficient complexing agent from licorice, glycyrrhizic acid, which is known to profoundly influence the therapeutic activity of the alkaloid in the complex. In MeOH, DMSO, or aqueous solutions, lappaconitine has been shown to form a stable complex with glycyrrhizic acid with 1:1 stoichiometry over a broad concentration range from 1 microM to 300 microM. The stability constant K(11) equals 2.0 x 10(5) M(-1) in aqueous solution. A similar complex of lappaconitine hydrobromide--the pharmaceutical formulation used in the treatment of arrhythmia--is 2 orders of magnitude less stable than pure lappaconitine. A notable decrease in the rate of the photoinduced electron-transfer reaction between lappaconitine in a complex with glycyrrhizic acid and tyrosine allows the suggestion of an explicit interrelation between the suppressed chemical reactivity of the bound alkaloid and the changes of its therapeutic efficiency.     

A Strict Inequality for the Random Triangle Model

The random triangle model on a graph G, is a random graph model where the usual i.i.d. measure is perturbed by a factor q ^t(), where q1 is a constant, and t() is the number of triangles in the random subgraph . Here we consider the case where G is the usual two-dimensional triangular lattice, for which there exists a percolation threshold p _c (q) such that the probability of getting an infinite connected component of retained edges is 0 for p<p _c (q), and 1 for p>p _c (q). It has previously been shown that p _c (q) is a decreasing function of q. Here we strengthen this by showing that p _c (q) is strictly decreasing. This confirms a conjecture by Häggström and Jonasson.