Ultrafiltration of a polymer-electrolyte mixture

We present a mathematical model to describe the ultrafiltration behaviour of polymer-electrolyte mixtures. The model combines the proper thermodynamic forces (pressure, chemical potential and electrical potential differences) with multicomponent diffusion theory. The model is verified with experimental data on the ultrafiltration of aqueous solutions of PEG-4000 and potassium phosphate. The single solute rejection of PEG-4000 goes through a maximum as also found by others. The single solute rejection of potassium phosphate depends on the ionic strength of the solution. At low ionic strength rejections are found of 50%. Solutions containing a high concentration of PEG-4000 and potassium phosphate show a negative rejection for potassium phosphate. This is caused by the strongly non-ideal behaviour of these aqueous solutions. The model predicts the behaviour of single solute experiments quite well, but some deviations are found with the mixed solute experiments. However, negative rejections found in the mixed solute experiments are predicted by the model.     

Genetic microheterogeneity of human transthyretin detected by IEF

Mutations of the human transthyretin (TTR) gene have attracted medical interest as a cause of amyloidosis. Recently, we have described in detail an electrophoretic procedure with PAGE followed by IEF in urea gradients for the study of the microheterogeneity of TTR monomers (Altland, K., Winter, P., Sauerborn, M. K., Electrophoresis 1999, 20, 1349-1364). In this paper, we present a study on 49 different mutations of TTR including 33 that result in electrically neutral amino acid substitutions. The aims of the investigation were to test the sensitivity of the procedure to detect TTR variants in patients with TTR amyloidosis and their relatives and to identify some common characteristics that could explain the amyloidogenicity of these variants. We found that all tested amyloidogenic mutations could be detected by our method with the exception of those for which the corresponding variant was absent in plasma samples. Most of the electrically neutral amyloidogenic TTR variants had in common a reduced conformational stability of monomers by the activity of protons and urea. For three variants, e.g. TTR-F64L, TTR-I107V and TTR-V122I, the monomers had a conformational stability close to that of normal monomers but we found experimental and structural arguments for a weakening of the monomer-monomer contact. All types of amyloidogenic mutations affected the stability of TTR tetramers.     

Separation and detection of individual submicron particles by capillary electrophoresis with laser-light-scattering detection

Separation and detection of individual submicron polystyrene spheres using capillary electrophoresis with laser-light-scattering detection has been demonstrated. Electrophoretically separated particles were passed through a focused laser beam and light scattered from individual particles was collected at 90 degrees. Each diameter of polystyrene spheres injected (from 110 to 992 nm) resulted in the observation of a well-defined migration window containing multiple peaks, each arising from the light scattered by an individual particle. The migration time window for individual particles of a particular size corresponded well to the migration time of a peak from a population of particles of the same size detected using a UV absorbance detector. The electrophoretic mobility and scattered light intensity were determined for each particle detected. The average scattered light intensity for each particle size was consistent with Mie scattering theory. Particles as small as 110 nm in diameter were detected individually using this method, but particles with a diameter of 57 nm could not be individually detected. The number of single particle scattering events was counted and compared to the theoretical number of particles injected electrokinetically, and the detection efficiency determined ranged from 38 to 57% for polystyrene spheres of different sizes. The laser-light-scattering detection method was directly compared to laser-induced fluorescence detection using fluorescent polystyrene microspheres. The number of particles detected individually by each method was in agreement.     

Construction of modified embedded atom method potentials for Cu, Pt and Cu-Pt and modelling surface segregation in Cu3Pt alloys

In this work, surface segregation to Cu₃Pt surfaces is studied with the modified embedded atom method (MEAM). This work is triggered by the catalytic importance of Cu-Pt alloys, together with the contradictory experimental results for the surface segregation in Cu₃Pt(1 1 1) alloys based on low energy ion scattering (LEIS) [Y.G. Shen, D.J. O’Connor, K. Wandelt, R.J. MacDonald, Surf. Sci. 328 (1995) 21] and low energy electron diffraction (LEED) [Y. Gauthier, A. Senhaji, B. Legrand, G. Tréglia, C. Becker, K. Wandelt, Surf. Sci. 527 (2003) 71]. In order to accurately describe the segregation behaviour in the Cu₃Pt system, a reliable potential, that is also applicable to surface phenomena, is indispensable. Therefore, first, new MEAM parameters are derived, consistently based on ab initio density functional theory (DFT) calculations, according to a method that is a modification of previous work [P. van Beurden, G.J. Kramer, Phys. Rev. B 63 (2001) 165106]. Upon testing, these parameters prove to reproduce very well various surface properties of this system. Next, Monte Carlo (MC) simulations combined with the newly derived MEAM potentials are set up to investigate surface segregation to low index single crystal surfaces. For the Cu₃Pt(1 1 1) surface, our MC/MEAM simulations agree completely with the available LEIS evidence and contradict the unusual depth profile based on LEED. However, the slight Pt enrichment observed in the LEED experiments can be reproduced by assuming a slight Pt excess in the bulk of the sample. The simulated composition depth profile, on the other hand, does not agree with the LEED evidence. Also, for the Cu₃Pt(1 0 0) surface, the MC/MEAM results agree completely with LEIS experiments. For the Cu₃Pt(1 1 0) surface, finally, the MC/MEAM simulations show a somewhat deviating behaviour with respect to the experimental LEIS evidence. The possibility of a missing-row reconstruction is evaluated, but cannot explain the discrepancy for the Cu₃Pt(1 1 0) system. In order to further investigate the deviation from the experiments, additional DFT and MEAM calculations are performed in search of the preferred surface termination for Cu₃Pt(1 1 0). Both DFT and MEAM calculations agree on the pure Cu layer as the most stable surface termination. Although the experiment was extensively tested for reproducibility, it possibly reflects a metastable state. Finally, in view of the importance of small and less orderly particles in catalysis, the newly derived MEAM parameters are used in order to study the segregation to Cu₃Pt vicinal surfaces with {1 1 1} terraces, for which no experimental information is available yet.     

Crystal structure predictions for acetic acid

Possible crystal structures of acetic acid were generated, considering eight space groups and assuming one molecule in the asymmetric unit. Our grid-search method was compared with a Monte Carlo approach as implemented in the Biosym/MSI Polymorph Predictor. This revealed no sampling deficiencies. A large number of possible crystal structures were found (∼100 within only 5 kJ/mol), including the experimental structure. Energy minimizations were done with a united-atoms force field (GROMOS), an all-atoms force field (AMBER), and a potential that describes the electrostatic interactions with distributed multipoles (DMA). In all cases, the experimental structure had a low lattice energy. The number of hypothetical crystal structures was reduced considerably by removing space-group symmetry constraints, or by a primitive molecular dynamics shake-up. Nevertheless, sufficient structures of equal or lower energy compared with the experimental structure remained to suggest that other factors need to be considered for genuine structure prediction. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 459–474, 1998     

Ab initio crystal structure predictions for flexible hydrogen‐bonded molecules. Part III. Effect of lattice vibrations

In crystal structure predictions possible structures are usually ranked according to static energy. Here, this criterion has been replaced by the free energy at any temperature. The effects of harmonic lattice vibrations were found by standard lattice‐dynamical calculations, including a rough estimate of the effects of thermal expansion. The procedure was tested on glycol and glycerol, for which accurate static energies had been obtained previously (Part II of this series). It was found that entropy and zero‐point energy give the largest contribution to free energy differences between hypothetical crystal structures, adding up to about 3 kJ/mol for the structures with lowest energy. The temperature‐dependent contribution to the energy and the effects of thermal expansion showed less variation among the structures. The overall accuracy in relative energies was estimated to be a few kJ/mol. The experimental crystal structure for glycol corresponded to the global free energy minimum, whereas for glycerol it ranked second at 1 kJ/mol. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 816–826, 2001     

Ab initio crystal structure predictions for flexible hydrogen‐bonded molecules. Part II. Accurate energy minimization

A method is described to perform ab initio energy minimization for crystals of flexible molecules. The intramolecular energies and forces are obtained directly from ab initio calculations, whereas the intermolecular contributions follow from a potential that had been parameterized earlier on highly accurate quantum‐chemical calculations. Glycol and glycerol were studied exhaustively as prototypes. Lists of hypothetical crystal structures were generated using an empirical force field, after which ab initio energy minimizations were performed for a few hundreds of these. The experimental crystal structures were found among the structures with lowest energy, provided that sufficiently large basis sets were used. Moreover, their crystal geometries were well reproduced. This approach enables a systematic comparison between the merits of force fields at various levels of sophistication. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 805–815, 2001     

Crystal structure predictions using five space groups with two independent molecules. The case of small organic acids

Crystal structure generations with two independent molecules have been performed for a series of carboxylic acids, using a slightly modified version of the OPLS force field. It was found that in this way the experimental structures with one independent molecule were produced as special cases, except for the molecules with four or more internal degrees of freedom. This work shows that a search with two independent molecules in only five space groups, although costly in computer power, can automatically also find structures with one independent molecule in many supergroups. Considering the observed abundances of structural classes, such a search should cover more than 95% of the possible homomolecular crystal structures.     

Helix formation by poly{bis(hydroxyethyl)-L-glutamine}

Circular dichroism spectra are reported for poly {bis(hydroxyethyl)-L-glutamine}. Its conformation is predominantly that of a statistical coil at or above ambient temperature in water. It becomes partially helical upon cooling. The helix-forming potential of this polypeptide in water is between that of poly(hydroxyethyl-L-glutamine) and poly(hydroxypropyl-L-glutamine). The polypeptide has a large helix content when the solvent is rich in trifluoroethanol.