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PubMed1997

Structural aspects of interfacial adsorption. A crystallographic and site-directed mutagenesis study of the phospholipase A2 from the venom of Agkistrodon piscivorus piscivorus.

Recent genetic and structural studies have shed considerable light on the mechanism by which secretory phospholipases A2 interact with substrate aggregates. Electrostatic forces play an essential role in optimizing interfacial catalysis. Efficient and productive adsorption of the Class I bovine pancreatic phospholipase A2 to anionic interfaces is dependent upon the presence of two nonconserved lysine residues at sequence positions 56 and 116, implying that critical components of the adsorption surface differ among enzyme species (Dua, R., Wu, S.-K., and Cho, W. (1995) J. Biol. Chem. 270, 263-268). In an effort to further characterize the protein residues involved in interfacial catalysis, we have determined the high resolution (1.7 A) x-ray structure of the Class II Asp-49 phospholipase A2 from the venom of Agkistrodon piscivorus piscivorus. Correlation of the three-dimensional coordinates with kinetic data derived from site-directed mutations near the amino terminus (E6R, K7E, K10E, K11E, and K16E) and the active site (K54E and K69Y) defines much of the interface topography. Lysine residues at sequence positions 7 and 10 mediate the adsorption of A. p. piscivorus phospholipase A2 to anionic interfaces but play little role in the enzyme's interaction with electrically neutral surfaces or in substrate binding. Compared to the native enzyme, the mutant proteins K7E and K10E demonstrate comparable (20-fold) decreases in affinity and catalysis on polymerized mixed liposomes of 1-hexadecanoyl-2-(1-pyrenedecanoyl)-sn-glycero-3-phosphocholine and 1,2-bis[12-(lipoyloxy)dodecanoyl]-sn-glycero-3-phosphoglycerol, while the double mutant, K7E/K10E, shows a more dramatic 500-fold decrease in catalysis and interfacial adsorption. The calculated contributions of Lys-7 and Lys-10 to the free energy of binding of A. p. piscivorus phospholipase A2 to anionic liposomes (-1.8 kcal/mol at 25 degrees C per lysine) are additive (i.e. -3.7 kcal/mol) and together represent nearly half of the total binding energy. Although both lysine side chains lie exposed at the edge of the proposed interfacial adsorption surface, they are geographically remote from the corresponding interfacial determinants for the bovine enzyme. Our results confirm that interfacial adsorption is largely driven by electrostatic forces and demonstrate that the arrangement of the critical charges (e.g. lysines) is species-specific. This variability in the topography of the adsorption surface suggests a corresponding flexibility in the orientation of the active enzyme at the substrate interface.

Adsorption↗
PubMed2004

On the role of CO2 laser treatment in the human serum albumin and human plasma fibronectin adsorption on zirconia (MGO-PSZ) bioceramic surface.

The nature of the surface strongly influences the composition and recognizability of the adsorbed protein layer, which in turn affects the subsequent cellular interactions. Thus, to understand the biological response to a material, especially in vitro, one must fully understand the nature of the adsorbed protein film that forms on the material. This study investigates the fundamental interactions between the human serum albumin (no-cell adhesive) and human plasma fibronectin and bioinert ceramic following CO(2) laser treatment. The analysis of the albumin and fibronectin adsorption was conducted on the untreated and CO(2) laser-modified magnesia partially stabilized zirconia (MgO-PSZ) bioceramic using an ellipsometry. It was found that the adsorptions of albumin and fibronectin were influenced by the surface properties. The albumin adsorption was affected by the surface roughness and wettability characteristics of the MgO-PSZ and decreased with these properties, while the fibronectin adsorption was increased with wettability characteristics and predominantly governed by this property. Moreover, the considerable change in the polar component of surface energy, gamma(sv) (p), and its effect on protein adsorption implied that the albumin and fibronectin adsorption on the MgO-PSZ surfaces was probably due to the polar and chemical interactions. The value of this work is to provide a novel technique and useful information for manipulating protein adsorption and thereof cellular interactions.

Biocompatible Materials↗
PubMed1996

Comparison of Adsorption and Conformation of Hemoglobin and Myoglobin on Various Inorganic Ultrafine Particles.

Human hemoglobin and horse myoglobin were adsorbed on the two types of ultrafine silica (silica 1 and 2), titania, and zirconia particles under various conditions. The order of the affinity of the ultrafine particles to these proteins was silica 1 < silica 2 < zirconia = titania under the same adsorption condition. The conformational changes in these proteins upon adsorption and desorption were monitored by both circular dichroism and absorbance spectra. In both spectrum measurements, a similar tendency was observed, indicating that the adsorption of proteins causes the changes not only in the secondary structure but also in the tertiary structure. In all the particles, the extent of conformational changes upon adsorption increased with decreasing pH and adsorption amount. On the other hand, the extent of conformational changes was significantly affected by the particle type (its order was silica 1 < silica 2 < zirconia = titania) under the same adsorption condition. The percentage of desorption and the extent of refolding of desorbed proteins were lower when the extent of conformational changes in the adsorbed states was higher. These results clearly demonstrate that higher affinity between particle surfaces and proteins causes larger conformational changes and consequently leads to higher irreversibility in adsorption and conformational states.

Journal Article↗
PubMed1997

Adsorption of alpha-Alanine on Boehmite

The adsorption of alpha-alanine on boehmite is effected at room temperature in aqueous solutions at different pH values (2.5, 6.0, 12.2). The different forms of alpha-alanine prevalent in the different media result in different modes of adsorption, different uptakes that generally increase with pH, and different responses of the pore structure of boehmite. The dominance of the zwitterionic form at pH 6.0 results in an adsorption mode that permits a three-dimensional array of crystalline alpha-alanine to be built on the surface of boehmite while allowing hydrogen bonding with surface hydroxyls through the carboxylate anion end, which acts as a proton acceptor. The formation of a well-crystallized alpha-alanine is less probable at pH 2.5, where the adsorption of the two prevailing forms, CH3CH(N+H3)COO- and CH3CH(N+H3)COOH, makes them unable to interact in a way that permits a three-dimensional framework of hydrogen bonding between COO- as proton acceptor and N+H3 as proton donor. At pH 12.2 the prevailing CH3CH(NH2)COO- is adsorbed coordinately on Lewis acid sites represented by the surface Al ions. Such a mode of adsorption is more disperse than that occurring at lower pH values, causing an increase in the uptake and noncrystallinity of adsorbed alanine. The narrow pores where surface hydroxyl density is high are most affected by adsorption at pH 2.5 and 6.0, which facilitates building the three-dimensional order, due to limited space, of adsorbed alanine. In alkaline medium the adsorption takes place mainly in larger pores, which results in a lower loss of area than expected. The results are compared with those obtained previously using glycine as the adsorbate, and the differences are discussed in terms of the characteristics of hydration in aqueous media.

Journal Article↗
PubMed1997

The Effect of the Ionic Strength on the Adsorption Isotherms of Nickel on Silica

Isotherms of adsorption of Ni(II) on silica from 0.01 M NaClO4 are almost linear with a log-log slope of 0.95, and the percentage of uptake at a given pH is insensitive to the initial Ni concentration. At higher ionic strengths (>/=0.1 mol dm-3 ) two kinds of behavior are observed. For low Ni concentrations the adsorption is approximately equal to that observed at lower ionic strengths. For higher Ni concentrations (but still much lower than the concentration of surface sites) the Ni adsorption from 0.1 and 0.3 mol dm-3 NaClO4 is lower than the adsorption from 0.01 M NaClO4 by a factor of 2 (pH 9) to 3 (pH 8). When water is replaced by a mixed solvent (e.g., 2% aqueous DMSO, THF, methanol, glycerol) the Ni adsorption isotherms obtained in the presence of 0.1 mol dm-3 NaClO4 are linear with a log-log slope of 0.95. These adsorption isotherms are rather insensitive to the nature and concentration of the organic cosolvent. It is impossible to explain the Ni adsorption curves over the entire studied range of initial concentrations and ionic strengths in terms of surface complexation model, unless the Boltzmann factor is properly corrected.

Journal Article↗
PubMed1997

Phosphate Adsorption on Hematite, Kaolinite, and Kaolinite-Hematite (k-h) Systems As Described by a Constant Capacitance Model

The constant capacitance model was used to describe phosphate adsorption on hematite, kaolinite, and a kaolinite-hematite system (k-h). The model assumes a ligand exchange mechanism and considers the charge on both adsorbate and adsorbent. The model is shown to provide a quantitative description of phosphate adsorption on these, including the effect of varying pH values. The computer program Ma-Za 2, a program that fits equilibrium constants to experimental data using an optimization technique, was used to obtain optimal values for the anion surface complexation constants on hematite, kaolinite, and a kaolinite-hematite system, while the PC program Ma-Za 1 in Q-Basic language was used for the application of the constant capacitance model. The model represented adsorption of phosphate anions well over the entire pH range studied (3.8-9.0). The main advantage of the model is its ability to represent changes in anion adsorption occurring with changes in pH. Extension of the model to describe phosphate adsorption in a mixed system, such as the kaolinite-hematite system, using the surface protonation-dissociation constant of hematite was qualitatively successful. In mixed system the model reproduced the shape of the adsorption isotherms well over the pH range 3.8-9.0. However, phosphate adsorption was overestimated. The hematite and the kaolinite-hematite system were synthesized and identified by X-ray, NMR, and FT-IR spectroscopy.

Journal Article↗
PubMed1997

Adsorption of Dextrin at Mineral/Water Interface

The adsorption mechanism of dextrin on aqueous minerals such as fluorite, apatite, galena, magnetite, gamma-alumina, and graphite was studied by adsorption experiments, zeta potential measurements, and FT-IR studies. Depending on the nature of the mineral surface, dextrin was found to interact in three different ways viz. by chemisorption, physisorption, or hydrophobic-hydrophobic interaction. The adsorption density of dextrin was found to be pH dependent. Maximum adsorption of dextrin was obtained around the pH at which the mineral surface is highly hydroxylated. The mechanism of dextrin interaction with the surface metal hydroxy sites, ( identical withMeOH), was found to proceed via chemical complexation. A linear relationship was observed between the adsorption density of dextrin and the pH of maximum surface hydroxylation. Zeta potential measurements have indicated the possibility of dextrin adsorption by electrostatic interaction under the conditions where mineral surface and dextrin are oppositely charged. Furthermore dextrin was found to adsorb on hydrophobic minerals such as graphite by hydrophobic-hydrophobic interaction. However, the magnitude of adsorption by electrostatic and hydrophobic interaction was found to be very marginal compared to that of chemical complexation. Copyright 1997 Academic Press. Copyright 1997Academic Press

Journal Article↗
PubMed1997

Surface Energetics of the Adsorption Process of a Cationic Dye on Leacril Fabrics

In this paper are presented data on the zeta potential, adsorption processes, and energy of interaction between Leacril and a cationic dye, crystal violet (CV) in the process of dyeing of Leacril. The method for obtaining the values of zeta potential of the system is the streaming potential technique. Previous models of bundles of capillaries have been tested by comparison with precise values of the zeta potential of the system. The model that presents a higher confidence level is the Goring and Mason model. The zeta potential results reveal that the uptake of crystal violet on Leacril fibers takes place by means of electrostatic attraction between the cation of the dye and both the sulfonate and the sulfate end-groups of the Leacril. Given the hydrophobic character of the Leacril and the amphiphilic nature of the dye molecules, hydrophobic attractions between the fiber and the hydrophobic part of the crystal violet might account for the adsorption of the cationic dye onto the fibers even when hindered by electrostatic repulsion. The data for the adsorption of the dye on the fibers indicate that the adsorption is favored by increasing the temperature of the process. This could be due to increased ionization of the sulfonate and sulfate end-groups of the Leacril, with increasing temperature of adsorption. The behavior of the components of the interaction energy, between the Leacril and the dye, is analyzed in the present paper in light of van Oss's theory. Using both the thin layer wicking and contact angle techniques, we have determined the values of the components of the surface-free energy of Leacril fabrics and of the crystal violet, respectively. The total interaction energy between the Leacril and the cationic dye has been obtained by means of sum of three components, the electrical, DeltaGEL, acid-base, DeltaGAB, and Lifshitz-van der Waals, DeltaGLW, respectively. Estimation of the electrical component makes use of the zeta potential of the system Leacril/cationic dye obtained by means of the streaming potential technique. Two approaches were followed in order to estimate the interfacial (excluding electrostatic) free energy of interaction DeltaGIF between Leacril fibers and CV: (i) the determination of the interactions between the fiber and dye solutions of different concentrations and (ii) estimations of DeltaGIF between fiber and dye molecules in the presence of water. These combined methods are in agreement with the experimental results obtained in this work. These methods explain qualitatively the adsorption of the cationic dye on Leacril in the entire range of concentrations of dye used in the present work. Based on the study of the interfacial interactions carried out in the present work the adsorption of crystal violet onto Leacril is favorable from a thermodynamic point of view. Copyright 1997 Academic Press. Copyright 1997Academic Press

Journal Article↗
PubMed1997

Non-Langmuir Adsorption on Planar Surfaces: Nonlinear Theory and Experiment

A theory of non-Langmuir kinetics of polymer molecules on planar surfaces in the framework of the kinetic-conformational-diffusive-controlled model has been developed. It is shown that an overshoot in both the equilibrium adsorption and the adsorption kinetics may be explained using the conformational kinetics of polymer molecules on planar surfaces with the term depending on the surface polymer concentration. From the experimental data, equations were derived to calculate the equilibrium constant, K0(p), the conformational parameter beta characterizing the conformational process, the thickness of the adsorbed layer, dad.layer, the weight fraction of polymers in the adsorbed layer, Wtplat, the activation energy of adsorption, (-DeltaH), the diffusion coefficient, D(theta), the activation energy of diffusion in the adsorbed layer, Q, the relaxation time tconf characterizing the conformational process, and the time needed to attain the equilibrium state for the adsorption process. The adsorption of poly(dimethylsiloxane) on an oxide surface of silicon has been investigated by ellipsometry. Overshoots in both the equilibrium adsorption and the adsorption kinetics of poly(dimethylsiloxane) on an oxide surface of a silicon were observed. Copyright 1997 Academic Press. Copyright 1997Academic Press

Journal Article↗
PubMed1998

Adsorption of Polyoxyethylenic Nonionic and Anionic Surfactants from Aqueous Solution: Effects Induced by the Addition of NaCl and CaCl2.

The adsorption processes of two different types of surfactant from aqueous solutions have been studied on several solids. The adsorbates used were a nonionic (TX-100) and a series of anionic (NP4S, NP10S, and NP25S) oxyethylenic surfactants. As adsorbent, five nonporous solids, including three quartz (QA, QB, and QC), a kaolin, and a dolomite, were chosen for this study, since these types of materials are frequently found in oil reservoirs. Great differences have been found in the adsorption plateaus, depending on the nature of the surfactant (anionic or nonionic). The influence of the presence of NaCl and CaCl2 in the solutions has been also studied. NaCl affects the adsorption of anionic surfactants on quartz and kaolin samples in a similar way. When this salt is added, the amount of anionic surfactants adsorbed on the solid surfaces increases. Some differences in the adsorption of the TX-100 surfactant, depending on the nature of the surface and the type of salt added, have been detected. Basically, three different adsorption behaviors have been found when NaCl is added. The amounts of TX-100 adsorbed decrease when NaCl is added to the solution on the QA sample; the amounts increase on the QB and kaolin samples; no alteration is observed on QC and dolomite samples. Changes in adsorption isotherms, depending on whether NaCl or CaCl2 is added, have also been observed. For the same five adsorbents, zeta potential measurements also have been carried out. When the nonionic TX-100 surfactant is adsorbed, a decrease in the negative values of the zeta potential takes place. However, for the adsorption of anionic surfactants, an increase in the negative values of the zeta potential is detected. The surface charge has been also determined by potentiometric titration (in presence and in absence of TX-100), and a decrease in surface charge when TX-100 is adsorbed on the sample surfaces has been detected. Copyright 1998 Academic Press.

Journal Article↗
PubMed1999

Development of Composite Adsorbents of Carbon and Intercalated Clay for N2 and O2 Adsorption: A Preliminary Study.

Composite adsorbents of carbon and alumina intercalated montmorillonite were prepared and characterized by adsorption of N2 and O2 at various temperatures. The effects of pyrolysis, temperature, heating rate, subsequent degassing, and doping of cations and anions were investigated. The adsorption capacities of the composite adsorbents developed at higher temperatures (0 and -79 degreesC) are found to be larger than those of normal alumina pillared clays. The experimental results showed that the framework of these adsorbents is made of alumina particles and clay sheets while the pyrolyzed carbon distributes in the space of interlayers and interpillars. The pores between the carbon particles, clay sheets, and alumina pillars are very narrow with very strong adsorption forces, leading to enhanced adsorption capacities at 0 and -79 degreesC. The composite adsorbents exhibit features similar to those of carbonaceous adsorbents. Their pore structures, adsorption capacities, and selectivities to oxygen can be tailored by a controlled degassing procedure. Meanwhile, ions can be doped into the adsorbents to modify their adsorption properties, as usually observed for oxide adsorbents like zeolite and pillared clays. Such flexibility in pore structure tailoring is a potential advantage of the composite adsorbents developed for their adsorption and separation applications. Copyright 1999 Academic Press.

Journal Article↗
PubMed1999

Charge Adjustments upon Adsorption of a Weak Polyelectrolyte to a Mineral Oxide: The Hematite-Humic Acid System.

The proton adsorption to a mixture of purified Aldrich humic acid (PAHA) and hematite is investigated. Basic insight into the charge adjustment process is obtained by using a self-consistent-field lattice theory for polyelectrolyte adsorption. The calculations indicate that upon adsorption the component with the highest initial charge density tends to induce charges on the other component. The number of induced charges can show a maximum when the surface charge and the charge of the segments in direct contact with the surface roughly balance each other. Experimentally, the humic acid-hematite system is investigated by proton titrations. The alterations in charge density caused by adsorption of PAHA to hematite are investigated by comparing the proton adsorption on the individual samples with that on their mixtures. Upon adsorption a part of the functional groups of humic acid forms complexes with some of the surface sites of hematite. This interaction reduces the proton binding to the humic acid at relatively low pH and it promotes the proton adsorption on the oxide surface at relatively high pH. Copyright 1999 Academic Press.

Journal Article↗
PubMed1999

A Mechanistic Approach to Modeling Single Protein Adsorption at Solid-Water Interfaces.

A kinetic model for single component protein adsorption which can be readily extended to adsorption from multi-protein solutions was developed, and used to simulate adsorption of site-directed, structural stability mutants of bacteriophage T4 lysozyme. The model allows for two different adsorbed "states," distinguished by different binding strengths and different occupied areas. The presence of an increasing energy barrier to adsorption was incorporated into the model by formulating the adsorption rate constants as functions of time. Numerical analysis was performed using the Marquardt method. Estimated model parameters were consistent with the effect of structural stability on adsorption. In particular, kinetic parameters were such that adsorption into the more tightly bound, conformationally altered state was favored by less stable variants. Copyright 1999 Academic Press.

Journal Article↗
PubMed2000

Competitive Adsorption of Ferricyanide and Ferrocyanide on gamma-Al(2)O(3) Surface.

In the past 3 decades, research has proven the significance of competitive adsorption in the equilibrium of pollutants between solid and liquid phases. However, studies on the competitive adsorption of complex ions are very limited in spite of its important role in transporting pollutants in the natural environment. The objective of this study is to derive the thermodynamic parameters of the competitive adsorption between ferricyanide and ferrocyanide from the modified Langmuir isotherm and the triple-layer model (TLM) to determine the location of adsorption. The effects of pH, temperature, and ion concentration on competitive adsorption onto gamma-Al(2)O(3) were investigated. The results demonstrate that ferrocyanide is more competitive than ferricyanide. By comparing the derived K(app) with K(int), we inferred that the adsorption of ferricyanide and ferrocyanide onto gamma-Al(2)O(3) was achieved through outer-sphere complexation. The negative DeltaH degrees indicated that the adsorption was exothermic. The positive entropy (Delta S degrees ) was caused by the replacement and release of a greater number of smaller surface ions by adsorbed ferricyanide and ferrocyanide ions of larger size. Copyright 2000 Academic Press.

Journal Article↗
PubMed2000

Effect of Particle Association on 2,2'-Bipyridyl Adsorption onto Kaolinite.

The effect of particle concentration, in kaolin suspensions, on the adsorption of 2,2'-bipyridyl was studied. Adsorption expressed in units of micromoles per gram decreased as a result of the increase in particle concentration and also as a result of the presence of coagulant (0.25 M NaCl). Dispersion treatment with sodium hexametaphosphate increased the adsorption of bipyridyl. The decrease in adsorption with the increase in particle concentration suggests a possible relation between adsorption and flocculation phenomena. On the basis of classic flocculation theory a straight-line relation was obtained between the square root of the adsorption maximum (mmol/L) and particle concentration (g/L). It is concluded that particle association, which is a function of particle concentration, reduces the surface/aqueous interface and consequently the adsorption of bipyridyl. Copyright 2000 Academic Press.

Journal Article↗
PubMed2000

Temperature Influence of Benzene Adsorption by a Microporous Silica.

The theory for volume filling of micropores is used to describe benzene adsorption isotherms measured over a 25 K temperature range. The adsorption potential or molar work of adsorption for the isotherm at 298 K is derived and compared with Weibull, Gaussian, and gamma potential distribution functions. The Weibull function is fitted via a two-term Dubinin-Radushkevich (D-R) equation. The closest data fit occurs for the gamma distribution. The two-term D-R potentials are interpreted as indicating adsorption by primary micropores followed sequentially by secondary micropores. Analysis of the distribution of adsorption enthalpy for the porous solid compared with a nonporous standard suggests that the predominant pore width is 1.2 nm. The interpretation of the differential molar adsorption entropy at 298 K suggests that strongly localized adsorption occurs in the primary micropores and two-dimensional translational motion with rotation in the plane of the ring occurs in the secondary micropores. Copyright 2000 Academic Press.

Journal Article↗
PubMed2000

Adsorption Behavior of Amino Acids on a Stainless Steel Surface.

The adsorption behavior of various amino acids on a stainless steel surface was investigated at 30 degrees C and over a pH range of 3-10. Acidic and basic amino acids except histidine adsorbed remarkably at pH 3-4 and 7-10, respectively, and showed Langmuir-type adsorption isotherms. The effects of pH and ionic strength on the adsorption isotherms were investigated to analyze the interactions between amino acids and adsorption sites on the stainless steel. Hydrophobic amino acids and glycine showed only small adsorbed amounts at all pHs tested. For the acidic and basic amino acids, reversibility of the absorption and the influence of the ionic strength on the adsorption behavior were examined. The adsorption isotherms of the derivatives of aspartic acid were also measured in order to examine the contribution of the carboxylic groups of acidic amino acids to the adsorption. Furthermore, a Fourier-transform infrared spectroscopic analysis and semiempirical molecular orbital calculation were carried out to analyze the ionization states and the configuration of the amino acids adsorbed on a stainless steel surface. These investigations suggest that the acidic and basic amino acids adsorb through two electrostatic interactions of two ionized groups in the amino acid with a stainless steel surface. Copyright 2000 Academic Press.

Journal Article↗
PubMed2000

Liquid-Phase Adsorption of Phenol onto Activated Carbons Prepared with Different Activation Levels.

The influence of the pore size distribution of activated carbon on the adsorption of phenol from aqueous solutions was explored. Activated carbons with different porous structures were prepared by gasifying a bituminous coal char to different extents of burn-off. The results of adsorption experiments show that the phenol capacity of these carbons does not proportionally increase with their BET surface area. This reflects the heterogeneity of the carbon surface for adsorption. The pore size distributions of these carbons, determined according to the Dubinin-Stoeckli equation, were found to vary with the burn-off level. By incorporating the distribution with the Dubinin-Radushkevich equation using an inverse proportionality between the micropore size and the adsorption energy, the isotherms for the adsorption of phenol onto these carbons can be well predicted. The present study has demonstrated that the heterogeneity of carbon surface for the phenol adsorption can be attributed to the different energies required for adsorption in different-size micropores. Copyright 2000 Academic Press.

Journal Article↗