Conversion of a cyclotriphosphazene to a cyclohexaphosphazene by ring expansion

Deprotonation of a cyclotriphosphazene with a tert-butylamino group in the side chain results in ring expansion to a very stable, planar cyclohexaphosphazene derivative that still contains eight P-Cl bonds suitable for forming macromolecular structures.     

Thermodynamic characterization of 3-[(3-cholamidopropyl)-dimethylammonium]-1-propanesulfonate (CHAPS) micellization using isothermal titration calorimetry: temperature, salt, and pH dependence

A systematic investigation of the micellization process of a biocompatible zwitterionic surfactant 3-[(3-cholamidopropyl)-dimethylammonium]-1-propanesulfonate (CHAPS) has been carried out by isothermal titration calorimetry (ITC) at temperatures between 278.15 K and 328.15 K in water, aqueous NaCl (0.1, 0.5, and 1 M), and buffer solutions (pH = 3.0, 6.8, and 7.8). The effect of different cations and anions on the micellization of CHAPS surfactant has been also examined in LiCl, CsCl, NaBr, and NaI solutions at 308.15 K. It turned out that the critical micelle concentration, cmc, is only slightly shifted toward lower values in salt solutions, whereas in buffer media it remains similar to its value in water. From the results obtained, it could be assumed that CHAPS behaves as a weakly charged cationic surfactant in salt solutions and as a nonionic surfactant in water and buffer medium. Conventional surfactants alike, CHAPS micellization is endothermic at low and exothermic at high temperatures, but the estimated enthalpy of micellization, ΔHM0, is considerably lower in comparison with that obtained for ionic surfactants in water and NaCl solutions. The standard Gibbs free energy, ΔGM0, and entropy, ΔSM0, of micellization were estimated by fitting the model equation based on the mass action model to the experimental data. The aggregation numbers of CHAPS surfactant around cmc, obtained by the fitting procedure also, are considerably low (nagg ≈ 5 ± 1). Furthermore, some predictions about the hydration of the micelle interior based on the correlation between heat capacity change, Δcp,M0, and changes in solvent-accessible surface upon micelle formation were made. CHAPS molecules are believed to stay in contact with water upon aggregation, which is somehow similar to the micellization process of short alkyl chain cationic surfactants.     

TETRABUTYLAMMONIUM－TETRACYANOQUINODIMETHANE AS ELECTRON－TRANSFER MEDIATOR IN AMPEROMETRIC GLUCOSE SENSOR

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Erratum to: Solution behavior of aqueous mixtures of low and high molecular weight hydrophobic amphiphiles

Mixtures of a hydrophobic triblock copolymer (L121, PEO₅PPO₆₈PEO₅) and a hydrophobic anionic surfactant (AOT, Sodium bis(2-ethylhexyl)sulfosuccinate), each alone forming turbid vesicular solutions in water, aggregate to produce a thermodynamically stable, transparent and isotropic solution. Mixed AOT/L121 aggregates could be confirmed by fluorescence, surface tension, differential scanning calorimetry (DSC) and isothermal titration calorimetry (ITC). In an isotropic region, where mixed aggregates are formed, there is a synergistic interaction between monomers of AOT and L121 in the mixture. In addition, Small Angle Neutron Scattering (SANS) experiments provided evidence that mixed aggregates have the shape of either spheres (with a certain polydispersity) or very short ellipsoids (axial ratio below 2), confirming a transition from giant multilamellar vesicles to small aggregates upon mixing the two hydrophobic amphiphiles. Upon dilution, the morphology changes to disk-like. From an examination of the results of all the methods the peculiar behavior of the mixed AOT/L121 system is explained.     

New Perspectives for Old Clusters: Anderson–Evans Anions as Building Blocks of Large Polyoxometalate Frameworks in a Series of Heterometallic 3 d–4 f Species

A series of nine [Sb₇W₃₆O₁₃₃Ln₃M₂(OAc)(H₂O)₈]^17? heterometallic anions ( Ln₃M₂ ; Ln=La–Gd, M=Co; Ln=Ce, M=Ni and Zn) have been obtained by reacting 3 d metal disubstituted Krebs‐type tungstoantimonates(III) with early lanthanides. Their unique tetrameric structure contains a novel {MW₉O₃₃} capping unit formed by a planar {MW₆O₂₄} fragment to which three {WO₂} groups are condensed to form a tungstate skeleton identical to that of a hypothetical trilacunary derivative of the ?‐Keggin cluster. It is shown, for the first time, that classical Anderson–Evans {MW₆O₂₄} anions can act as building blocks to construct purely inorganic large frameworks. Unprecedented reactivity in the outer ring of these disk‐shaped species is also revealed. The Ln₃M₂ anions possess chirality owing to a {Sb₄O₄} cluster being encapsulated in left‐ or right‐handed orientations. Their ability to self‐associate in blackberry‐type vesicles in solution has been assessed for the Ce₃Co₂ derivative.     

Thermodynamic characterization of different actin isoforms

Summary The thermodynamic properties of the cardiac and skeletal a-actin isoforms were studied to characterize the molecular bases of the functional differences between them with the method of differential scanning calorimetry (DSC). The thermal properties of the actin filaments were described in the presence of calcium and magnesium ions as well. Based on the calculated free energy changes the α-cardiac actin filaments appeared to be more stable in its physiologically more relevant, magnesium saturated form. The magnesium saturated form of the α-cardiac actin filaments seemed to be more stable compared to the calcium saturated form of it. The enthalpy and entropy changes could differentiate between the α-cardiac and α-skeletal actin isoforms and between the calcium and magnesium saturated cardiac actin isoforms as well. Our results can demonstrate that the few differences between the amino acid sequences of the α-actin isoforms have an influence on the thermal properties and maybe on the function of these proteins as well.     

Structure of starch-bentonite gels

Mixed gels of starch and bentonite are investigated in the interval 0.056–0.089 of total solids/water ratio and 0-100% starch in the solids. The bentonite was a sodium calcium bentonite with a Na/Ca ratio of 1.76. In water it forms gels consisting of a network of band-type structures. Starch forms gels through hydrogen bonds between granules and/or amylose and amylopectin present on the external surfaces of granules and/or in fully stretched form. Mixed gels of bentonite and starch were obtained by adding corn starch granules to the already formed bentonite gels and heating the mixture above the Kofler gelatinization temperature. Amylose and amylopectin were adsorbed on strands of band-type structures of mont-morillonite lamellae. Starch gellation, e.g. diffusion of amylose out of the granule, was facilitated in the presence of bentonite. On the other hand, the presence of starch favored delamination of the montmorillonite particle into thinner lamellae. Maximum gelatinization and polymer adsorption were observed for gels with 20% starch and 80% bentonite. Montmorillonite networks formed the continuous phase for 0-80% starch. At higher starch concentrations, montmorillonite flakes were dispersed within the polymer network. Increase in the water content of the gels caused segregation of the bentonite and starch.     

Asymptotic quadratic convergence of the parallel block-Jacobi EVD algorithm with dynamic ordering for Hermitian matrices

The proof of the asymptotic quadratic convergence is provided for the parallel two-sided block-Jacobi EVD algorithm with dynamic ordering for Hermitian matrices. The discussion covers the case of well-separated eigenvalues as well as clusters of eigenvalues. Having p processors, each parallel iteration step consists of zeroing 2p off-diagonal blocks chosen by dynamic ordering with the aim to maximize the decrease of the off-diagonal Frobenius norm. Numerical experiments illustrate and confirm the developed theory.     

Retention of configuration in the nucleophilic substitution reactions of some nine-membered ansa derivatives of cyclotriphosphazatriene

X-ray crystallographic evidence shows that nucleophilic substitution reactions of two different types of cyclophosphazene derivatives with relatively rigid nine-membered ansa rings leads to the first demonstration of retention of configuration in these molecular systems.     

Anomalous NMR behavior of meso compounds with remote stereogenic centers on addition of chiral shift reagent or chiral solvating agent

A problem has arisen in using chiral shift reagents (CSR) and chiral solvating agents (CSA) to determine meso and racemic forms of diastereoisomers in which the stereogenic centers of the molecules are separated by achiral spacers. It is found that NMR signals of both meso and racemic forms of diastereoisomers may exhibit doubling on addition of CSR/CSA, which means that unequivocal assignments cannot be made without characterizing the effects for separate meso and racemic forms; this is particularly important for additions of CSR/CSA at relatively low concentrations, which always result in the splitting of some NMR signals of diastereoisomers. The phenomenon is demonstrated in the (31)P NMR spectra of meso and racemic forms of three spermine-bridged gem-disubstituted cyclotriphosphazatrienes, 1a-c, and compared with analogous achiral molecules, the per-substituted spermine-bridged cyclotriphosphazatrienes 2a-d. As expected, only one set of (31)P NMR signals was observed for the achiral compounds 2a-d, even on addition of CSA. Two sets of (31)P NMR ABX multiplets corresponding to meso and racemic diastereoisomers were observed for compounds 1a-c; on addition of CSA, the signals of at least one of the multiplets for each compound separated into more than the expected groups of three lines with an intensity distribution of 2:1:1. To understand this phenomenon, the meso and racemic forms of 1a and 1b and the meso form of 1c have been separated and characterized by X-ray crystallography. On addition of CSA to the racemic forms of 1a and 1b, the (31)P NMR spectrum shows the expected doubling of signals, but, unexpectedly, the same is observed for each of the meso forms of 1a-c. Analogous results using both CSA and CSR have been obtained for the meso and racemic forms of the diastereoisomeric piperazine-bridged macrocyclic-phosphazene compound, 3, whereas no effect was observed for the two meso forms of the doubly bridged macrocyclic-phosphazene compound 4. The phenomenon of doubling of the (31)P NMR signals of the meso form of singly bridged cyclotriphosphazatrienes, 1a-c and 3, is explained by consideration of the equilibrium in solution of independent complexation of a chiral ligand with molecules that have two chiral cyclophosphazene moieties separated by an achiral spacer group. The results show that the stereogenicity of such diastereoisomeric molecules in solution cannot be characterized unequivocally by NMR measurements on addition of either CSR or CSA.