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  • 1
    Electronic Resource
    Electronic Resource
    Modeling of phase separation in PEG-salt aqueous two-phase systems (1996)
    Hoboken, NJ : Wiley-Blackwell
    AIChE Journal 42 (1996), S. 3508-3522 
    Details
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: A molecular model based on the integral equation theory of statistical thermodynamics is used to study phase separation in PEG-salt aqueous two-phase systems. PEG molecules are modeled as hard spheres that attract each other through a temperaturedependent Yukawa potential, which mimics the effect of PEG-water hydrogen bonding on the attraction between PEG molecules. The salt ions are modeled as charged hard spheres interacting through a Coulombic potential. Excess thermodynamic properties due to Coulombic and Yukawa interactions are calculated by analytical solutions to the Ornstein-Zernike equation for the mean spherical approximation closure. Yukawa parameters for PEG-PEG interactions are determined by fitting the theoretical phase diagram for a pure Yukawa fluid to the experimental phase diagram for a PEG-water mixture. The model predicts experimentally observed trends: increasing the temperature increases the slope and length of the tie lines; increasing the PEG molecular weight increases the miscibility gap; and increasing the anion charge lowers the salt concentration at which phase separation occurs. Theoretical results allow us to infer the relative importance of ion-PEG interactions, ion-solvent interactions, and the interpenetrable nature of PEG molecules on the phase separation in PEG-salt aqueous two-phase systems.
    Additional Material: 12 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/aic.690421220
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  • 2
    Electronic Resource
    Electronic Resource
    Theory of precipitation of protein mixtures by nonionic polymer (1992)
    Hoboken, NJ : Wiley-Blackwell
    AIChE Journal 38 (1992), S. 573-591 
    Details
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: A theory is developed to predict the solubility of protein mixtures in solutions containing nonionic polymer. Effective protein-protein interactions due to polymer are taken to be volume-exclusion potentials derived using statistical mechanics. Statistical-mechanical perturbation theory is used to calculate chemical potentials. The effects of protein size, mole fraction and polymer concentration on solubility are explored. The theory is extended to include electrostatic interactions. The excess chemical potential of the proteins due to the charges on all species is calculated using the mean spherical approximation for a mixture of charged hard spheres. The theory predicts: the larger protein is preferentially precipitated over the smaller one; the more concentrated protein is more likely to precipitate; and increasing the charge of a particular protein reduces its ability to precipitate.
    Additional Material: 14 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/aic.690380411
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  • 3
    Electronic Resource
    Electronic Resource
    Computer simulation of protein refolding pathways and intermediates (1995)
    Hoboken, NJ : Wiley-Blackwell
    AIChE Journal 41 (1995), S. 985-990 
    Details
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Computer simulation studies of refolding pathways and the formation of intermediates for a simple, 2-D lattice protein model are presented. The sequence of the 20-bead model protein chain is chosen so that hydrophobic beads will reside in the protein interior in the native state. Nonbonded hydrophobic beads attract each other with strength ∊ decreasing the |∊/kT| mimics increasing the concentration of the denaturant. Dynamic Monte Carlo simulations and exhaustive conformational searches have been performed on an isolated model protein sequence at different levels of |∊| (different denaturant concentrations). As the denaturant is withdrawn, the model protein exhibits a transition from a random coil state to a compact native state with a hydrophobic core. The refolding process is observed to be cooperative in that the chain does not start folding until the middle section has folded correctly, and proceeds along preferred pathways that are populated by distinct, partially folded intermediates.
    Additional Material: 9 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/aic.690410428
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  • 4
    Electronic Resource
    Electronic Resource
    Prediction of gas adsorption in 5a zeolites using Monte Carlo simulation (1991)
    Hoboken, NJ : Wiley-Blackwell
    AIChE Journal 37 (1991), S. 769-779 
    Details
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Adsorption of air in 5A zeolites was studied using Monte Carlo simulations in the grand canonical ensemble (μ, V, T constant). Site-site potentials were used to model the adsorbate-zeolite and adsorbate-adsorbate interactions. The potential model contains one adjustable parameter that was fit to a single experimental isotherm data point. Adsorption isotherms and heats of adsorption were determined for pure argon, oxygen, and nitrogen at 203.15 K, 233.15 K, and 297.15 K from 0.1 bar to 4.0 bar. Multicomponent adsorption isotherms were determined for binary mixtures of oxygen and nitrogen at 203.15 K. The results for the pure-component isotherms are in excellent agreement with experimental data. The results for the heat of adsorption are in good agreement with experimental data for argon and oxygen, but not for nitrogen. The results for multicomponent adsorption isotherms are qualitatively correct; however, the simulation was not able to quantitatively predict mixture data.
    Additional Material: 13 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/aic.690370514
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  • 5
    Electronic Resource
    Electronic Resource
    Statistical-mechanical model of protein precipitation by nonionic polymer (1990)
    Hoboken, NJ : Wiley-Blackwell
    AIChE Journal 36 (1990), S. 1517-1528 
    Details
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: A theoretical approach to predict the solubility of proteins in solutions containing nonionic polymers is presented. The effective protein-protein interaction due to the presence of the polymer is related to the volume-exclusion potential of Asakura and Oosawa. Statistical-mechanical perturbation theory, as originally applied by Gast et al. to model colloidal flocculation, is used to calculate free energies, from which solubility curves for varying protein-polymer diameter ratios are obtained. The theory correctly predicts all the trends observed in experimental studies of these systems. To explain the influence of processes parameters such as the pH and the ionic strength on protein solubility, the intermolecular potential is improved by the addition of an electrostatic interaction term. It is found that theoretical predictions of the variation in protein solubility, both with the solution pH and the ionic strength, are in accordance with experimental observations.
    Additional Material: 13 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/aic.690361007
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  • 6
    Electronic Resource
    Electronic Resource
    Solute excluded-volume effects on the stability of globular proteins: A statistical thermodynamic theory (1996)
    New York : Wiley-Blackwell
    Biopolymers 38 (1996), S. 273-284 
    Details
    ISSN: 0006-3525
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A statistical thermodynamic theory is developed to investigate the effects of solute excluded volume on the stability of globular proteins. Proteins are modeled as two states in chemical equilibrium: the denatured state is modeled as a flexible chain of tangent hard spheres (pearl-necklace chain) while the native state is modeled as a single hard sphere. Study of model proteins bovine pancreatic trypsin inhibitor and lysozyme in a McMillan-Mayer model solution of hard spheres indicates that the excluded volume of solutes has three distinct types of effects on protein stability: (1) small-size solutes strongly denature proteins, (2) medium-size solutes stabilize proteins at low solute concentrations and destabilize them at high concentrations, and (3) large-size solutes stabilize native-state proteins across the whole liquid region. The study also finds that increasing the chain length of hard-chain polymer solutes has an effect on protein stability that is similar to increasing the diameter of spherical solutes. This work qualitatively explains why stabilizers tend to be large size molecules such as sugars, polymers, polynols, nonionic, and anionic surfactants while denaturants tend to be small size molecules such as alcohols, glycols, amides, formamides, ureas, and guanidium salts. Quantitative comparison between theoretical predictions and experimental results for folding free energy changes shows that the excluded-volume effect is at least as important as the binding and/or electrostatic effects on solute-assisted protein-denaturation processes. Our theory may also be able to explain the effect of excluded volume on the Φ condensation of DNA. © 1996 John Wiley & Sons, Inc.
    Additional Material: 7 Ill.
    Type of Medium: Electronic Resource
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  • 7
    Electronic Resource
    Electronic Resource
    Effect of solvent conditions upon refolding pathways and intermediates for a simple lattice protein (1997)
    New York : Wiley-Blackwell
    Biopolymers 42 (1997), S. 399-409 
    Details
    ISSN: 0006-3525
    Keywords: protein folding ; folding pathways ; folding intermediates ; computer simulation ; solvent effects ; Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Folding pathways and intermediates for a two-dimensional lattice protein have been investigated via computer simulation at various denaturant concentrations. The protein is represented as a chain of 8 hydrophobic (H) and 12 polar (P) beads on a square lattice sequenced in such a way that the native state is a compact hydrophobic core surrounded by a shell of polar beads. Two nonbonded H beads are said to attract each other with a potential of mean force of strength ε. Increasing |ε/kT| mimics decreasing the denaturant concentration in the solution. Dynamic Monte Carlo simulations have been performed in order to investigate the folding transition and the folding pathways. Sharp folding - unfolding transitions are observed and the folding process proceeds along well-defined pathways that are populated by partially folded intermediates. The folding pathways as well as the populations of the intermediates are strongly dependent upon the denaturant concentration. Generally, intermediates containing long open stretches of H beads are more populated at high denaturant concentration, whereas compact intermediates containing a substantial number of hydrophobic contacts are more populated at low denaturant concentrations. The folding process is also observed to be cooperative in nature in that the chain does not start folding until a key fold in the middle section of the chain is formed correctly. © 1997 John Wiley & Sons, Inc. Biopoly 42: 399-409, 1997
    Additional Material: 10 Ill.
    Type of Medium: Electronic Resource
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