Multiple grid methods for classical molecular dynamics

Presented in the context of classical molecular mechanics and dynamics are multilevel summation methods for the fast calculation of energies/forces for pairwise interactions, which are based on the hierarchical interpolation of interaction potentials on multiple grids. The concepts and details underlying multigrid interpolation are described. For integration of molecular dynamics the use of different time steps for different interactions allows longer time steps for many of the interactions, and this can be combined with multiple grids in space. Comparison is made to the fast multipole method, and evidence is presented suggesting that for molecular simulations multigrid methods may be superior to the fast multipole method and other tree methods.     

Synthesis of new stable analogues of prostacyclin: (±)-6a-oxo-6,9-metrano-15-hydroxyprosta-5,13-dienoic acids

The synthesis is described of (±)-6a-oxo-6,9-methano-15-hydroxyprosta-5,13-dienoic acids as stable analogues of prostacyclin with blood platelet aggregation inhibiting activity.     

A fast algorithm for estimating large-scale permeabilities of correlated anisotropic media

The problem of estimating large-scale permeabilities of reservoirs based on knowledge of the small-scale permeabilities is addressed. We present an accurate and fast algorithm to calculate the global permeabilities of two- or three-dimensional correlated and anisotropic block samples, thus providing a fast algorithm for obtaining grid block permeabilities for reservoir simulators from small scale data. The algorithm is tested on both two- and three-dimensional tube networks generated from real images and fractal forgeries modeling porous media. In almost all cases, the algorithm estimates the correct global permeability (calculated using exact but slow algorithms) of the network to better than 5%. The new algorithm is comparable in speed to conventional averaging techniques, such as the geometric mean, but the obtained estimates are always much better.List of Symbols K permeability of network (global permeability - K _e estimated permeability - K K×(L_x×L_y)/L_z - K permeability perpendicular to layering - K permeability parallel with layering - L _x, L_y, L_z Network size inx, y andz-directions - L Size of quadratic (cubic) network - Q global flux through network - U Q/(L_x×L_y), Darcy velocity (flux per unit area) - V volume of network - P pressure drop across network - a,b parameters in Equation (4) - P _i pressure at sitei - q _ij flux between nodesi andj - parameter of Pareto distribution - porosity - K(i) permeability at site (block)i - K _ij permeability of bond between nodesi andj - K _min minimumk(i) for sample - K _max maximumK(i) for sample - fluid viscosity     

Die massenspektrometrische retro-Diels-Alder-Reaktion: 1, 2, 3, 4-Tetrahydroisochinolin und 1, 2, 3, 4-Tetrahydronaphthalin (Tetralin). 31. Mitteilung über massenspektrometrische Untersuchungen

The Mass Spectral retro-Diels-Alder-Reaction: 1,2,3,4-Tetrahydroisoquinoline and 1,2,3,4-Tetrahydronaphthaline (Tetraline) The retro-Diels-Alder reaction of 1,2,3,4-tetrahydroisoquinoline and of its N-acetyl derivative was confirmed on the basis of labelled derivatives (Scheme 2). Furthermore, the loss of ethylene was investigated with the 1,2,3,4-tetrahydronaphthalene- and 1,2,3,4-tetrahydronaphthalen-1-one-derivatives given in Schemes 4, 5 and 6. In the case of the 1,2,3,4-tetrahydronaphthalen-1-one-derivatives ethylene is lost via a retro-Diels-Alder reaction. The loss of ethylene from 1,2,3,4-tetrahydronaphthalene ( 1 ) and from its derivatives is a rather complex reaction (Scheme 8): 1/3 of ethylene is split off 1 ⁺ via a formal retro-Diels-Alder reaction, 2/3 are lost after a specific rearrangement. The ratio of these two fragmentation pathways depends very much on the substituents placed at the aliphatic and the aromatic rings, compare e.g. Table 4.     

Conformational sampling of hydrocarbon and lipid chains in an orienting potential

A Distribution Biased Monte Carlo (DBMC) sampling procedure is developed for the efficient generation of chain conformations in the oriented environment of lipid membranes and other liquid crystalline systems. Conformations of the sn-1 chain of dipalmitoyl phosphatidylcholine (DPPC) were generated by independently sampling torsion angles from continuous distributions in an orienting potential based on a Marcelja mean field; depending on the chain position, the convergence in the deuterium order parameters (S_CD ) was 100 to 3000 times more efficient with DBMC than with Brownian dynamics. Optimization using joint distribution and torsional potentials of mean force yielded a further threefold increase in sampling efficiency. Overall chain tilt was included using Euler angle rotations and a separate field strength for the anchor. A segmental DBMC procedure was used to generate a set of complete DPPC conformations with well-converged conformationally averaged S_CD consistent with experimental values. These conformations show considerable flexibility, not only in the hydrocarbon tails, but additionally in both the glycerol and head-group portions of the lipid. An appendix compares DBMC with a number of other Monte Carlo and stochastic dynamics algorithms using the example of a bistable oscillator, and illustrates the tuning of parameters for optimal convergence. © 1994 by John Wiley & Sons, Inc.

1 This article is a US Government work and, as such, is in the public domain in the United States of America.

     

Synthesis and application of single-isomer 6-mono(alkylimidazolium)-beta-cyclodextrins as chiral selectors in chiral capillary electrophoresis

Novel single isomers of positively charged beta-CDs were prepared via nucleophilic substitution of 6-monotosyl-beta-CD with alkylimidazoles to afford 6-mono(alkylimidazolium)-beta-CD tosylates and then 6-mono(alkylimidazolium)-beta-CD chlorides by anion exchange. The chiral resolution abilities of these cationic CDs were studied by CE using dansyl (Dns)-amino acids as model analytes. From the experimental results, it was found that both resolution and selectivity of these cationic CDs were dependent on the following parameters: concentration of chiral selectors, pH of the running buffer, counteranions of the CDs, side chain length of the n-alkyl-imidazolium cation, temperature of the capillary column, and organic modifier used. The concentration of chiral selectors required for enantioseparation varied from 3 to 30 mM. The BGE pH played an important role in the resolution of Dns-amino acids. For acidic BGEs, chiral resolution increased with pH (4.0-6.0) and reached a local maximum at pH 6.0. However, better resolutions were obtained with basic phosphate buffer at pH 9.6. Methanol was found to be an effective organic modifier for the resolution of Dns-amino acids by CE.     

A Two-Step Synthesis of Camphosultam

Modifications of the known route to camphosultam led to an easy two-step process. The unique intermediate is simply isolated by filtration. The overall yield is 66% on a hundred gram scale.