Structure and dynamics of beta-cyclodextrin in aqueous solution at the density-functional tight binding level

Structure and dynamics of beta-cyclodextrin (beta-CyD), a prototype host for inclusion compounds of biological interest, is investigated by means of density-functional based tight-binding molecular dynamics (MD) simulations. The computational protocol is benchmarked against available experimental data and first-principles calculations. Solvent-solute interactions, including the diffusion into and dwell time of the solvent in the cavity of beta-CyD, are studied with a hybrid QM/MM method. Comparison of MD simulations of beta-CyD in the gas phase and in water shows that the solvent reduces the flexibility of the structure framework, while the terminal hydroxyl groups become more flexible and are embedded in a network of hydrogen bonds. Our 160 ps MD simulations, provide enough sampling to discuss the dynamics of the water inside the cavity. The dwell time of the encapsulated water molecule has a wide distribution with a peak at 70 fs. Surprisingly, despite only the 17% difference between the "top" and "bottom" opening area of the beta-CyD cone, 64% of the water molecules enter the cavity through the slightly bigger "bottom" aperture.     

Radical cascade transformations of tris(o-aryleneethynylenes) into substituted benzo[a]indeno[2,1-c]fluorenes

Oligomeric o-aryleneethynylenes with three triple bonds undergo cascade radical transformations in reaction with a Bu 3SnH/AIBN system. These cascades involve three consecutive cycle closures with the formation of substituted benzo[ a]indeno[2,1- c]fluorene or benzo[1,2]fluoreno[4,3- b]silole derivatives. The success of this sequence depends on regioselectivity of the initial attack of the Bu 3Sn radical at the central triple bond of the o-aryleneethynylene moiety. The cascade is propagated through the sequence of 5-exo-dig and 6-exo-dig cyclizations which is followed by either a radical attack at the terminal Ar substituent or radical transposition which involves H-abstraction from the terminal TMS group and 5-endo-trig cyclization. Overall, the transformation has potential to be developed into an approach to a new type of graphite ribbons.     

Weighted Korn inequalities for thin-walled elastic structures

Appropriate weighted norms in H¹ are presented such that the Korn type inequality is asymptotically sharp with respect to relative thickness and stiffness of the elastic plates. The weights depend crucially on the geometric structure of the plates' junction. To cite this article: O.V. Izotova et al., C. R. Mecanique 334 (2006).     

Variational calculation of some S-states of Coulomb three-body systems

A generalized Hylleraas-type basis set with three nonlinear parameters is proposed to study three-body systems interacting via coulomb forces within the framework of non-relativistic quantum mechanics. This basis set improves the rate of convergence with respect to previous ones, specially for non-symmetric systems and excited states of two electron atoms. Accurate binding energies and other properties for S-states of helium-like ions, muonic molecules and the positronium negative ion are reported. Received 21 July 2000 and Received in final form 4 October 2000     

Poly(ethylene glycol)–polyethyleneimine NanoGel™ particles: novel drug delivery systems for antisense oligonucleotides

A novel type of drug delivery system, termed NanoGel™ is proposed. NanoGel™ represent particles of a hydrophilic polymer network that were synthesized by cross-linking of polyethyleneimine (PEI) and carbonyldiimidazole-activated poly(ethylene glycol) (PEG) using emulsification/solvent evaporation technique. The resulting NanoGel™ was fractionated by gel-permeation chromatography. A major fraction with an average particle size of 120 nm was used in further experiments. Antisense phosphorothioate oligonucleotides (SODN) specific to human mdr1 gene were incorporated in these NanoGel™ particles. Loading of NanoGel™ particles with SODN resulted in reduction of the particle effective diameter to 80 nm and decreased zeta-potential due to neutralization of the charge of PEI chains by SODN. Accumulation of SODN incorporated in NanoGel™ particles in multidrug resistant (MDR) human oral epidermoid carcinoma cells (KBv) was significantly increased compared to the free SODN. Furthermore, efficient transport of SODN-loaded NanoGel™ particles across polarized monolayers of human intestinal epithelial cells (Caco-2) was demonstrated. Finally, antisense SODNs incorporated in NanoGel™ particles were found to effectively inhibit expression of P-glycoprotein (P-gp) efflux pump in MDR cell lines.     

Rate of convergence of a convolution-type estimator of the marginal density of a MA(1) process

In this paper moving-average processes with no parametric assumption on the error distribution are considered. A new convolution-type estimator of the marginal density of a MA(1) is presented. This estimator is closely related to some previous ones used to estimate the integrated squared density and has a structure similar to the ordinary kernel density estimator. For second-order kernels, the rate of convergence of this new estimator is investigated and the rate of the optimal bandwidth obtained. Under limit conditions on the smoothing parameter the convolution-type estimator is proved to be -consistent, which contrasts with the asymptotic behavior of the ordinary kernel density estimator, that is only -consistent.     

A stability criterion via fluid limits and its application to a polling system

We introduce a generalized criterion for the stability of Markovian queueing systems in terms of stochastic fluid limits. We consider an example in which this criterion may be applied: a polling system with two stations and two heterogeneous servers. This revised version was published online in June 2006 with corrections to the Cover Date.     

Theoretical Study of Glycidol Hyperbranching Polymerisation

Bulk hyperbranched anionic polymerisation of glycidol initiated by methoxide anion has been studied at B3PW91/6‐311+G(2d,p)//HF/6‐31G/d′) and MP2/6‐31+G(d′)//MP2/6‐31+G(d′) levels of theory. The Isodensity Polarised Continuum model was applied to take into account the environment effect. The results of molecular modelling are in good agreement with experimental data. According to the calculations glycidol polymerisation is thermodynamically controlled. The attack on the unsubstituted glycidol end is thermodynamically preferred except for the very first polymerisation step. Fast proton exchange reaction takes place during the polymerisation giving rise to the branching reaction. While the linear growth is caused by primary alkoxide attack, the branching reaction occurs by secondary alkoxide attack on the next glycidol molecule. It was found that cyclisation reactions show higher activation energies and are less favourable thermodynamically compared to polymerisation.     

A Novel,One‐Pot Synthesis of Novel 3F, 5F,and 8F Aromatic Polymers

A series of novel, linear, soluble, high‐molecular‐weight, fluorinated aromatic polymers has been obtained for the first time using a superacid‐catalyzed polyhydroxyalkylation reaction of fluorinated carbonyl‐containing compounds: 1,1,1,‐trifluoroacetone ( 1 ), 2,2,2‐trifluoroacetophenone ( 2 ), 2,3,4,5,6,‐pentafluorobenzaldehyde ( 3 ), and octafluoroacetophenone ( 4 ) with aromatic hydrocarbons such as biphenyl ( a ), phenyl ether ( b ), terphenyl ( c ), and 4,4′‐diphenoxybenzophenone ( d ). These Friedel‐Crafts‐type aromatic electrophilic substitution reactions are performed at room temperature in trifluoromethane sulfonic acid or in its mixtures with dichloromethane. The polymers obtained are soluble in common organic solvents, and colorless transparent films could be cast from the solutions. ¹H and ¹³C NMR analyses of the polymers synthesized reveal their linear, highly regular structure. The polymers also possess high thermostability.

     

Computational study of metathesis degradation of rubber, 1. Distribution of cyclic oligomers via intramolecular metathesis degradation of cis‐polybutadiene

The results of the molecular modelling of the intramolecular metathesis degradation of cis‐polybutadiene (cis‐PB) into cyclic oligomers at PM3 level of theory showed that the chain‐ring equilibrium is completely shifted towards the all‐trans cyclic isomers. The formation of cyclic products, containing from three to six butadiene units, from larger rings is thermodynamically favoured with cyclic butadiene tetramers and pentamers being the main products. These results are in reasonable agreement with most of the available experimental data. The discrepancy observed in some cases between found and calculated ring distributions for the intramolecular metathesis degradation of cis‐PB suggests that the reaction is kinetically controlled under certain conditions.