Small Mixed Clusters Research Articles

Molecular Dynamics Simulations of Essential Oil Ingredients Associated with Hyperbranched Polymer Drug Carriers.

Our work concerns the study of four candidate drug compounds of the terpenoid family, found as essential oil ingredients in species of the Greek endemic flora, namely carvacrol, p-cymene, γ-terpinene, and thymol, via the simulation method of molecular dynamics. Aquatic solutions of each compound, as well as a solution of all four together in realistic (experimental) proportions, are simulated at atmospheric pressure and 37 °C using an OPLS force field combined with TIP3P water. As verified, all four compounds exhibit a strong tendency to phase-separate, thereby calling for the use of carrier molecules as aids for the drug to circulate in the blood and enter the cells. Systems of two such carrier molecules, the hyperbranched poly(ethylene imine) (HBPEI) polyelectrolyte and hyperbranched polyglycerol (HPG), are examined in mixtures with carvacrol, the most abundant among the four compounds, at a range of concentrations, as well as with all four compounds present in natural proportions. Although a tendency of the terpenoids to cluster separately persists at high concentrations, promising association effects are observed for all drug–polymer ratios. HBPEI systems tend to form diffuse structures comprising small mixed clusters as well as freely floating polymer and essential oil molecules, a finding attributed to the polymer–polymer electrostatic repulsions, which here are only partially screened by the counterions. On the other hand, the electrically neutral HPG molecules cluster together with essential oil species to form a single nanodroplet. Currently, terpenoid–polymer clusters near lipid bilayer membranes are being studied to determine the propensity of the formed complexes to enter cell membranes.

Infrared Spectra of HCl−H2O Clusters in Helium Nanodroplets

Small (HCl)m(H2O)n clusters have been assembled in He droplets, and their spectra in the HCl stretch range (2500−3000 cm−1) have been obtained. In a recent He droplet study, a band at 2670 cm−1 was assigned to the dissociated H3O+(H2O)3Cl− ion pair. In this work, we have revised the assignment of this band to a cyclic hydrogen-bonded form of the (HCl)2(H2O)2 cluster based on careful measurements of the pickup pressure dependence as well as the transition moment angles associated with the HCl stretch vibrations. A number of vibrational bands due to small mixed clusters have also been observed. As the number of the captured water molecules increases, a broad feature appears that spans the 2550−2800 cm−1 range. The possible origin of this spectral broadening in large (HCl)m(H2O)n clusters is discussed.

Tight-binding molecular-dynamics study of amorphous carbon deposits over silicon surfaces

We report in this paper a procedure to simulate the deposit of carbon diamondlike films over the Si(001) surface. We use the method of tight-binding molecular dynamics and well-known transferable C-C and Si-Si interactions. We propose for the Si-C interaction a weighted average of the single element interactions and test it by studying crystalline SiC, the molecule SiC, some small mixed clusters and the surfaces of $\ensuremath{\beta}$ SiC(001). Results of the deposition simulation for low carbon concentrations are presented, showing the formation of a thin mixed interface layer, and its structural and some electronic properties are described.

Casein sub-micelles: do they exist?

The model of the casein micelle as a roughly spherical, fairly swollen particle of about 100 nm diameter with a hairy outer layer, is now generally accepted. Several workers have further assumed that the core of the micelle is built of sub-micelles, i.e., roughly spherical units of about 14 nm diameter, which are fairly tightly aggregated. It was originally assumed that the sub-micelles are held together in the micelle by bridges consisting of calcium phosphate. However, several studies, notably by Holt, have made this concept untenable. This also has made the existence of sub-micelles doubtful. Some years ago, however, van Dijk proposed that in the process of milk synthesis, casein molecules associate into small mixed clusters and that subsequently calcium phosphate is deposited into these particles, which would make them more dense (containing less solvent) and of a lower charge. Consequently, the particles would lose their colloidal stability and aggregate to form casein micelles. This concept agrees well with electron microscopic observations, notably by Buchheim, on milk formation in the lactating cell. It would also fit Brownian dynamics and molecular dynamics simulations of aggregation of small particles exhibiting weak colloidal attraction. Moreover, it readily fits several indications for a dynamic equilibrium between casein micelles and sub-micelles, for which examples of these will be given.

ChemInform Abstract: Synthesis and Reactivity of Small Mixed Clusters Formed by Transition Metals and Main Group Elements

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Quantum mechanical study of the structure and stability of molecular van der Waals' clusters II: (NH3)nHCN

The structures and stabilities of small mixed clusters of ammonia with hydrogen cyanide have been studied usingab initio electronic structure calculations at the Hartree Fock and Moller Plesset levels of theory. Stable structures for (NH3)nHCN, n=1, 2, 3 have been located. Two stable structures and a transition state for the heterodimer are described.

Synthesis and reactivity of small mixed clusters formed by transition metals and main group elements

Synthesis and reactivity of small mixed clusters formed by transition metals and main group elements