Search results for "Dynamic"

Helicate Extension as a Route to Molecular Wires

2008

We describe the preparation of a helicate containing four closely spaced, linearly arrayed copper(I) ions. This product may be prepared either directly by mixing copper(I) with a set of precursor amine and aldehyde subcomponents, or indirectly through the dimerization of a dicopper(I) helicate upon addition of 1,2-phenylenediamine. A notable feature of this helicate is that its length is not limited by the lengths of its precursor subcomponents: each of the two ligands wrapped around the four copper(I) centers contains one diamine, two dialdehyde, and two monoamine residues. This work thus paves the way for the preparation of longer oligo- and polymeric structures. DFT calculations and elec…

Understanding Noncovalent Interactions of Small Molecules with Carbon Nanotubes

2017

We combine experimental methods, density functional theory (DFT) calculations, and molecular dynamics (MD) simulations in the quantitative analysis of noncovalent interactions between (6,5)-enriched single-walled carbon nanotubes (SWNTs), as hosts, and a set of pyrene derivatives with different electronic properties and surface areas, as guests. The experiments and calculations were carried out in two solvents with markedly different polarities, namely 1,1',2,2'-tetrachloroethane (TCE) and N,N-dimethylformamide (DMF). Our results show that dispersion forces govern the supramolecular association of small molecules with (6,5)-SWNTs, with negligible contributions from ground-state charge-trans…

Protein Flexibility and Preorganization in the Design of Enzymes. The Kemp Elimination Catalyzed by HG3.17

2015

A recently designed enzyme, HG3.17, obtained by directed evolution, has shown a catalytic activity close to natural enzymes. Hybrid QM/MM molecular dynamics simulations for the Kemp elimination in this new enzyme have provided a deep insight into the origin of its catalytic efficiency. In this case, we have first demonstrated the presence of different conformations with significantly different reactivity. The larger reactivity is related with a better electrostatic preorganization of the environment that creates a more favorable electrostatic potential for the reaction to proceed. In HG3.17, efforts to improve the catalytic properties must be focused in possible mutations increasing the pre…

THEORETICAL INSIGHTS ON O2 AND CO ADSORPTION ON NEUTRAL AND POSITIVELY CHARGED GOLD CLUSTERS

2006

With the aim of understanding the elementary steps governing the oxidation of CO catalyzed by dispersed or supported gold nanoclusters, the adsorption of molecular species, such as O2 and CO, on model neutral and positively charged clusters (Au(n)(m+) n = 1, 9, and 13; m = 0, 1, and 3) has been studied using an ab initio approach. The computed structural and thermodynamic data related to the binding process show that molecular oxygen interacts better with neutral clusters, acting as an electron acceptor, while CO more strongly binds to positively charged species, thus acting as an electron donor.

Thermodynamics of copolymer solutions: how the pair interactions contribute to the overall effect.

2014

Vapor pressure measurements were performed for solutions of poly(methyl methacrylate-ran-tert-butyl methacrylate) with different weight fractions of tert-butyl methacrylate units, and their parental homopolymers in chloroform at 323 K, over a large domain of concentrations. The Flory–Huggins interaction parameters obtained from these experimental investigations show complex dependences of the Flory–Huggins interaction parameter on concentration and copolymer composition. This behavior can be modeled by taking into account an approach which considers the ability of the polymers to rearrange in a response to changes in their molecular surroundings [Adv. Polym. Sci. 2011, 238, 1–66]. According…

Liquid/Gas and Liquid/Liquid Phase Equilibria of the System Water/Bovine Serum Albumin

2013

The thermodynamic behavior of the system H2O/BSA was studied at 25 °C within the entire composition range: vapor pressure measurements via head space sampling gas chromatography demonstrate that the attainment of equilibria takes more than one week. A miscibility gap was detected via turbidity and the coexisting phases were analyzed. At 6 °C the two phase region extends from ca. 34 to 40 wt % BSA; it shrinks upon heating. The polymer rich phase is locally ordered, as can be seen under the optical microscope using crossed polarizers. The Flory-Huggins theory turns out to be inappropriate for the modeling of experimental results. A phenomenological expression is employed which uses three adju…

1988

An apparatus for turbidimetric measurements of demixing temperatures under shear flow is presented, and the results of experiments with trans-decahydronaphthalene/polystyrene (TD/PS) solutions, investigating molecular weights ranging from 100 kg/mol to 1770 kg/mol, are compared with viscometric data. It is found that the sign and magnitude of shear effects depend on molecular weight (Mw), polymer concentration (c2), and shear rate (). For the first time, it was possible to study a solution which exhibits shear dissolution at low shear rates but shear demixing at high shear rates.

Solvent quality as reflected in concentration- and temperature-dependent Flory-Huggins interaction parameters

2001

Flory-Huggins interaction parameters (χ) between poly(dimethylsiloxane) (weight-average molecular weight = 152 kg/mol) and various solvents (methyl ethyl ketone, toluene and n-octane) were determined as a function of composition and temperature with vapor-pressure measurements. These data, complemented by independent information for dilute and very concentrated solutions, serve as the basis for a discussion of solvent quality via different theoretical relations. Regardless of polymer concentration, the χ values fall from methyl ethyl ketone via toluene to n-octane, the ketone being the worst solvent and the hydrocarbon being the best solvent. The variation of χ with composition and temperat…

Interfacial tension between polymer-containing liquids - Predictability and influences of additives

1999

The first part of the contribution deals with the interfacial tension, σ, of phase-separated polymer solutions in single or mixed solvents and of binary polymer blends as a function of the relative distance to the critical temperature of the system, special attention being paid to the possibilities of theoretical prediction. Two methods are discussed in more detail. One is based on a realistic description of the Gibbs energy of mixing as a function of composition, the second correlates σ with the length of the measured tie line. The second part is devoted to another aspect, namely the effects of additives on the interfacial tension between the coexisting phases of demixed polymer solutions …

Interaction of pHPMA–pLMA Copolymers with Human Blood Serum and Its Components

2013

Immediately after administration, polymer therapeutics are exposed to complex biological media like blood which may influence and alter their physicochemical properties due to interactions with proteins or serum components. Among such interactions those leading to larger sized aggregates can be sensitively detected by dynamic light scattering (DLS) as a pre in vivo screening method. Random copolymers from N-(2-hydroxypropyl)methacrylamide and lauryl methacrylate p(HPMA-co-LMA) and copolymers loaded with the model drug domperidone were characterized by DLS in isotonic salt solution and in blood serum. The bare amphiphilic copolymer micelles (Rh=30 nm in isotonic salt solution) formed large a…