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J M Prausnitz

Publications and source records attributed to J M Prausnitz.

At least 19 recordsLinked to original sources
PubMed2006

Three-dimensional lattice Monte Carlo simulations of model proteins. IV. Proteins at an oil-water interface.

Lattice Monte Carlo simulations describe the adsorption of protein-like heteropolymer chains at an oil/water interface. The heteropolymers are designed sequences of 27 and 64 amino acid-type lattice sites taken from a 20-letter alphabet. We use our recently suggested energy scale to model oil and water. We investigate the effect of the oil parameters on adsorption properties of a single chain and on the aggregation of adsorbed chains while keeping the water parameters fixed to their optimum values found previously. By varying the oil parameters, we can cause a large range of adsorption behavior: from no adsorption to reversible adsorption to irreversible adsorption. We compare adsorption at a liquid/solid interface to that at a liquid/liquid interface. A liquid interface leads to stronger adsorption and denaturation than a solid interface with the same water and oil interaction parameters. We propose "optimal" oil parameters and use them to study multichain adsorption at a liquid interface.

Adsorption↗
PubMed2006

Sorption kinetics and equilibrium uptake for water vapor in soft-contact-lens hydrogels.

A gravimetric-sorption technique was used to obtain kinetic and equilibrium adsorption/desorption data for water vapor in four different soft-contact-lens (SCL) polymers at 35 degrees C. The SCL materials are a conventional hydrogel (polymacon) with a low water content at saturation (<50 wt %); two conventional hydrogels (hilafilcon A and alphafilcon A) with a high water content at saturation (>50 wt %); and a siloxane hydrogel (balafilcon A). Absorption and desorption equilibrium isotherms (water activity versus water weight fraction) overlap at high water contents, whereas significant hysteresis is observed at low water contents. The hysteresis loop is likely due to trapping of water in the polymer during the desorption process because of a rubber-to-glass transition of the SCL-film surfaces. Sorption data were interpreted using Flory-Rehner theory. The positive Zimm and Lundberg cluster function suggests that water tends to cluster in these SCL materials, except at very low water content. For polymacon and hilafilcon A, Fickian diffusion is observed for all activities for both water sorption and desorption. However, for alphafilcon A and balafilcon A, non-Fickian features appear at intermediate/low activities, in particular during water desorption, suggesting coupling of the diffusion process with polymer-matrix relaxation. The diffusion coefficient increases significantly with water concentration for polymacon and hilafilcon A (from approximately 0.3 x 10(-8) to 4.0 x 10(-8) cm2/s) because of augmented mixture free volume induced by water sorption, whereas a more complex composition dependence is observed for alphafilcon A and balafilcon A probably as consequence of a combined effect of polymer relaxation, plasticization, and water clustering.

Adsorption↗
PubMed2004

Phase behavior of aqueous solutions containing dipolar proteins from second-order perturbation theory.

Due to the interplay of Coulombic repulsion and attractive dipolar and van der Waals interactions, solutions of globular proteins display a rich variety of phase behavior featuring fluid-fluid and fluid-solid transitions that strongly depend on solution pH and salt concentration. Using a simple model for charge, dispersion and dipole-related contributions to the interprotein potential, we calculate phase diagrams for protein solutions within the framework of second-order perturbation theory. For each phase, we determine the Helmholtz energy as the sum of a hard-sphere reference term and a perturbation term that reflects both the electrostatic and dispersion interactions. Dipolar effects can induce fluid-fluid phase separation or crystallization even in the absence of any significant dispersion attraction. Because dissolved electrolytes screen the charge-charge repulsion more strongly than the dipolar attraction, the ionic strength dependence of the potential of mean force can feature a minimum at intermediate ionic strengths offering an explanation for the observed nonmonotonic dependence of the phase behavior on salt concentration. Inclusion of correlations between charge-dipole and dipole-dipole interactions is essential for a reliable calculation of phase diagrams for systems containing charged dipolar proteins and colloids.

Journal Article↗
PubMed2003

3D-Lattice Monte Carlo simulations of model proteins. Size effects on folding thermodynamics and kinetics.

Recently, we devised an energy scale to vary systematically amino-acid residue-solvent interactions for Monte Carlo simulations of lattice-model proteins in water. For 27-mer proteins, the folding behavior varies appreciably with the choice of interaction parameters. We now perform similar simulations with 64-mers to study the size dependence of the optimal energy parameter set for representing realistic behavior typical of many real proteins (i.e. fast folding and high cooperativity for single chains). We find that 64-mers are considerably more stable and more cooperative compared to 27-mers. The optimal interfacial-interaction-energy parameter set, however, is relatively size independent.

Amino Acid Sequence↗
PubMed2002

Effect of secondary structure on the potential of mean force for poly-L-lysine in the alpha-helix and beta-sheet conformations.

Because poly-L-lysine (PLL) can exist in the alpha-helix or beta-sheet conformation depending on solution preparation and solution conditions, PLL is a suitable candidate to probe the dependence of protein interactions on secondary structure. The osmotic second virial coefficient and weight-average molecular weight are reported from low-angle laser-light scattering measurements for PLL as a function of NaCl concentration, pH, and alpha-helix or beta-sheet content. Interactions between PLL molecules become more attractive as salt concentration increases due to screening of PLL charge by salt ions and at low salt concentration become more attractive as pH increases due to decreased net charge on PLL. The experimental results show that interactions are stronger for the beta-sheet conformation than for the alpha-helix conformation. A spherically-symmetric model for the potential of mean force is used to account for specific interactions not described by DLVO theory and to show how differences in secondary structure affect PLL interactions.

Circular Dichroism↗
PubMed2002

Protein-protein interactions in concentrated electrolyte solutions.

Protein-protein interactions were measured for ovalbumin and for lysozyme in aqueous salt solutions. Protein-protein interactions are correlated with a proposed potential of mean force equal to the free energy to desolvate the protein surface that is made inaccessible to the solvent due to the protein-protein interaction. This energy is calculated from the surface free energy of the protein that is determined from protein-salt preferential-interaction parameter measurements. In classical salting-out behavior, the protein-salt preferential interaction is unfavorable. Because addition of salt raises the surface free energy of the protein according to the surface-tension increment of the salt, protein-protein attraction increases, leading to a reduction in solubility. When the surface chemistry of proteins is altered by binding of a specific ion, salting-in is observed when the interactions between (kosmotrope) ion-protein complexes are more repulsive than those between the uncomplexed proteins. However, salting-out is observed when interactions between (chaotrope) ion-protein complexes are more attractive than those of the uncomplexed proteins.

Computer Simulation↗
PubMed2002

Hydrophobic forces between protein molecules in aqueous solutions of concentrated electrolyte.

Protein-protein interactions have been measured for a mutant (D101F) lysozyme and for native lysozyme in concentrated solutions of ammonium sulfate at pH 7 and sodium chloride at pH 4.5. In the mutant lysozyme, a surface aspartate residue has been replaced with a hydrophobic phenylalanine residue. The protein-protein interactions of D101F lysozyme are more attractive than those of native lysozyme for all conditions studied. The salt-induced attraction is correlated with a solvation potential of mean force given by the work required to desolvate the part of the protein surfaces that is buried by the protein-protein interaction. This work is proportional to the aqueous surface-tension increment of the salt and the fractional non-polar surface coverage of the protein. Experimental measurements of osmotic second virial coefficients validate a proposed potential of mean force that ascribes the salt-induced attraction between protein molecules to an enhancement of the hydrophobic attraction. This model provides a first approximation for predicting the protein-protein potential of mean force in concentrated aqueous electrolyte solutions; this potential is useful for determining solution conditions favorable for protein crystallization.

Chemical Precipitation↗
PubMed2001

Cloud-point temperatures for lysozyme in electrolyte solutions: effect of salt type, salt concentration and pH.

Liquid-liquid phase-separation data were obtained for aqueous saline solutions of hen egg-white lysozyme at a fixed protein concentration (87 g/l). The cloud-point temperature (CPT) was measured as a function of salt type and salt concentration to 3 M, at pH 4.0 and 7.0. Salts used included those from mono and divalent cations and anions. For the monovalent cations studied, as salt concentration increases, the CPT increases. For divalent cations, as salt concentration rises, a maximum in the CPT is observed and attributed to ion binding to the protein surface and subsequent water structuring. Trends for sulfate salts were dramatically different from those for other salts because sulfate ion is strongly hydrated and excluded from the lysozyme surface. For anions at fixed salt concentration, the CPT decreases with rising anion kosmotropic character. Comparison of CPTs for pH 4.0 and 7.0 revealed two trends. At low ionic strength for a given salt, differences in CPT can be explained in terms of repulsive electrostatic interactions between protein molecules, while at higher ionic strength, differences can be attributed to hydration forces. A model is proposed for the correlation and prediction of the CPT as a function of salt type and salt concentration. NaCl was chosen as a reference salt, and CPT deviations from that of NaCl were attributed to hydration forces. The Random Phase Approximation, in conjunction with a square-well potential, was used to calculate the strength of protein-protein interactions as a function of solution conditions for all salts studied.

Algorithms↗
PubMed2000

Interactions of proteins in aqueous electrolyte solutions from fluorescence anisotropy and circular-dichroism measurements.

Understanding aqueous protein-protein interactions is crucial for the development of a molecular-thermodynamic model for salt-induced protein precipitation. In addition, protein interactions are important in many disease states, including cataract formation and alpha-amyloid diseases. Fluorescence anisotropy provides a means to measure intermolecular interactions. In this work, monomer-dimer equilibrium of the peptide T4 LYS(11-36) was studied by fluorescence anisotropy over the pH range 4-7 and the NaCl concentration range 0.0-1.0 M, in a 25 mM sodium phosphate buffer. This 26 amino-acid peptide is derived from the beta-sheet region of the T4 lysozyme molecule and has the potential to form amyloid fibrils. The association constant for dimerization increases with rising pH and ionic strength. The potential of mean force for peptide-peptide interactions was calculated from these association constants. Circular-dichroism measurements show that the peptide becomes more structured as the pH rises, possibly contributing to increased association.

Amino Acid Sequence↗
PubMed2000

Interaction between oppositely charged micelles or globular proteins.

Monte Carlo simulations and the hypernetted chain theory are used to study the interaction between spherical macroions of opposite charge immersed in a solution of monovalent or divalent simple electrolyte. These calculations represent the first step toward studying phase behavior and precipitation kinetics in solutions containing a mixture of macroions with positive and negative net charges. The potential of mean force between colloidal particles is determined as a function of colloid-colloid separation. In addition to having an opposite sign, the calculated potential of mean force is found to be stronger and longer-ranged than observed in the case of equally charged macroparticles. The difference is more pronounced in the presence of divalent counterions and is especially noticeable when we compare distinct Coulombic and hard-core collision contributions to the interaction between equally and oppositely charged colloids. The present observations suggest the dominance of attractive forces between globally neutral but electrostatically heterogeneous macroparticles. While our numerical results cannot be successfully analyzed by existing theories, they provide useful guidance and benchmark data for the development of advanced analytic descriptions.

Chemical Precipitation↗
PubMed1998

Interaction between like-charged colloidal spheres in electrolyte solutions.

How colloidal particles interact with each other is one of the key issues that determines our ability to interpret experimental results for phase transitions in colloidal dispersions and our ability to apply colloid science to various industrial processes. The long-accepted theories for answering this question have been challenged by results from recent experiments. Herein we show from Monte-Carlo simulations that there is a short-range attractive force between identical macroions in electrolyte solutions containing divalent counterions. Complementing some recent and related results by others, we present strong evidence of attraction between a pair of spherical macroions in the presence of added salt ions for the conditions where the interacting macroion pair is not affected by any other macroions that may be in the solution. This attractive force follows from the internal-energy contribution of counterion mediation. Contrary to conventional expectations, for charged macroions in an electrolyte solution, the entropic force is repulsive at most solution conditions because of localization of small ions in the vicinity of macroions. Both Derjaguin-Landau-Verwey-Overbeek theory and Sogami-Ise theory fail to describe the attractive interactions found in our simulations; the former predicts only repulsive interaction and the latter predicts a long-range attraction that is too weak and occurs at macroion separations that are too large. Our simulations provide fundamental "data" toward an improved theory for the potential of mean force as required for optimum design of new materials including those containing nanoparticles.

Journal Article↗
PubMed1998

Protein-protein and protein-salt interactions in aqueous protein solutions containing concentrated electrolytes.

Protein-protein and protein-salt interactions have been obtained for ovalbumin in solutions of ammonium sulfate and for lysozyme in solutions of ammonium sulfate, sodium chloride, potassium isothiocyanate, and potassium chloride. The two-body interactions between ovalbumin molecules in concentrated ammonium-sulfate solutions can be described by the DLVO potentials plus a potential that accounts for the decrease in free volume available to the protein due to the presence of the salt ions. The interaction between ovalbumin and ammonium sulfate is unfavorable, reflecting the kosmotropic nature of sulfate anions. Lysozyme-lysozyme interactions cannot be described by the above potentials because anion binding to lysozyme alters these interactions. Lysozyme-isothiocyanate complexes are strongly attractive due to electrostatic interactions resulting from bridging by the isothiocyanate ion. Lysozyme-lysozyme interactions in sulfate solutions are more repulsive than expected, possibly resulting from a larger excluded volume of a lysozyme-sulfate bound complex or perhaps, hydration forces between the lysozyme-sulfate complexes.

Animals↗
PubMed1998

High-frequency alternating-crossed-field gel electrophoresis with neutral or slightly charged interpenetrating networks to improve DNA separation.

Toward improving DNA separations, this work reports the effects of high-frequency square-wave AC fields superimposed perpendicular to the direct current (DC) separation field on DNA migration in both polyacrylamide-based interpenetrating networks (IPNs) and in agarose networks. Compared to standard polyacrylamide gels, IPNs allow the separation of larger DNA (9000 bp vs. 5000 bp at 5 V/cm). In novel polyacrylamide-based IPNs, an alternating current (AC) field of 5 Hz increased the maximum DNA size separable. This effect was extended to larger DNA sizes with increasing electric-field strength up to and apparently beyond the power supply-limited maximum electric-field strength of 48 V/cm. The orthogonal AC field also increased mobility. These two results combine to yield a reduction in separation time of up to a factor of 20 in novel polyacrylamide-based IPNs. When negatively charged acrylic-acid groups were incorporated into the IPNs, the use of the AC field changed the DNA-network interaction, which altered the size dependence of DNA mobility. In agarose gels, an AC field of 50 Hz increased the size range separable; however, there was no increase in DNA mobility. There was no change in size dependence of mobility in an AC field when the number of charged groups in the agarose network was increased. Based on results in the literature, possible mechanisms were examined for the effects of the AC field on DNA separation.

Acrylic Resins↗
PubMed1997

Capillary electrophoresis of DNA restriction fragments: effect of polymer properties.

The mechanism of DNA separation by dilute polymer solutions in capillary electrophoresis is not well understood. To provide information on the effect of polymer properties on DNA separations, four polymers that differ in size, shape and stiffness were examined. Hydroxyethylcellulose of high molecular weight provides excellent separation of large DNA fragments (2027 bp - 23,130 bp). Polyvinylpyrrolidone separates DNA from 72 bp to 23 kbp; star-poly(ethylene oxide) and linear poly(ethylene oxide) provide separation of fragments to 1353 bp.

Base Composition↗
PubMed1997

Interactions of lysozyme in concentrated electrolyte solutions from dynamic light-scattering measurements.

The diffusion of hen egg-white lysozyme has been studied by dynamic light scattering in aqueous solutions of ammonium sulfate as a function of protein concentration to 30 g/liter. Experiments were conducted under the following conditions: pH 4-7 and ionic strength 0.05-5.0 M. Diffusivity data for ionic strengths up to 0.5 M were interpreted in the context of a two-body interaction model for monomers. From this analysis, two potential-of-mean-force parameters, the effective monomer charge, and the Hamaker constant were obtained. At higher ionic strength, the data were analyzed using a model that describes the diffusion coefficient of a polydisperse system of interacting protein aggregates in terms of an isodesmic, indefinite aggregation equilibrium constant. Data analysis incorporated multicomponent virial and hydrodynamic effects. The resulting equilibrium constants indicate that lysozyme does not aggregate significantly as ionic strength increases, even at salt concentrations near the point of salting-out precipitation.

Ammonium Sulfate↗
PubMed1993

Structural and catalytic properties of enzymes in reverse micelles.

Structural and catalytic properties of two enzymes--alpha-chymotrypsin and horse liver alcohol dehydrogenase (LADH)--are studied in bis(2-ethylhexyl) sodium sulfosuccinate (AOT)-isooctane reverse-micelle solutions. Circular dichroism (CD) and electron paramagnetic resonance spectroscopy (EPR) studies show little change in alpha-chymotrypsin structure upon incorporation into reverse micelles. These structural properties explain, in part, the observed activity of these two enzymes in reverse micelles. alpha-Chymotrypsin retains activity in reverse micelles and, in some cases, displays enhanced activity. A sixfold increase in the turnover number was observed in w0 = 10 reverse micelles. LADH has low activity in reverse micelles compared to that in aqueous solution. At w0 = 70, the turnover number of LADH is 18% of the aqueous value. Active-site titrations show a decrease in active enzyme concentration for both enzymes upon incorporation into reverse micelles. Little change in the structure of both LADH and alpha-chymotrypsin is observed with change of water content in the reverse-micelle system.

Alcohol Dehydrogenase↗
PubMed1988

Cholesterol aggregation and interaction with cholesterol oxidase in supercritical carbon dioxide.

High-pressure EPR spectroscopy indicates that cholesterol forms aggregates in supercritical carbon dioxide. In pure carbon dioxide, changes in cholesterol aggregate size or packing structure are observed with changing pressure. Near the critical point of carbon dioxide, cholesterol solubility is too low to permit significant aggregation, and monomeric cholesterol is observed. Addition of small amounts of dopants to supercritical carbon dioxide strongly affects cholesterol aggregation. Branched butanols (2-methyl-1-propanol and 2-methyl-2-propanol) and ethanol (to a lesser degree) promote cholesterol aggregation, while methanol, acetone, and 1-butanol do not. Cosolvents that promote aggregation also increase the rate at which cholesterol oxidase from Gloeocysticum chrysocreas catalyzes the oxidation of cholesterol. In supercritical carbon dioxide solutions, the EPR spectroscopy reveals little or no conformational change in cholesterol oxidase as 2-methyl-2-propanol or methanol is added. Damp cholesterol oxidase binds multiple cholesterol molecules; dry enzyme loses the ability to bind cholesterol. When molecular oxygen is the oxidizing agent, the rate of enzymatic cholesterol oxidation is greatly reduced in bone-dry carbon dioxide compared to that in water-saturated carbon dioxide.

3-Hydroxysteroid Dehydrogenases↗