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Physicochemical Properties of Protein-Smectite and Protein-Al(OH)x-Smectite Complexes
Abstract
A B S TRACT: Proteins (catalase, albumin, pepsin and lysozyme with different molecular weights and isoelectric points) were differently adsorbed at pH 7.0 on the clay fraction of three raw Nasaturated smectites (Crook and Uri montmorillonites and one hectorite). The adsorption isotherms of proteins on clay minerals showed typical Langmuir characteristics. Lysozyme was adsorbed under the effect of electrostatic interactions between the opposite charges of clay surfaces and protein molecules, whereas catalase and albumin were adsorbed under the effect of non-electrostatic forces. Pepsin was held in relatively high amounts only on the surfaces of hectorite. Proteins were intercalated in the interlayers spaces of smectites, usually undergoing extensive unfolding. Proteinsmectite complexes showed different behaviour to heating treatment. Some complexes remained practically unchanged after heating at 200~ Presence of 'wrecks' of interlayered materials was found after heating at 500~ for two hours. The amounts of proteins adsorbed on the external and interlamellar surfaces of clay minerals, partially coated with OH-A1 species, were much lower than those fixed on the clean clays. Only lysozyme was intercalated in chlorite-like complexes. In the last 40 years, adsorption of proteins on clay minerals has received the attention of many researches with diverse interests. The capacity of proteins to be sorbed is influenced by pH of the system, surface area of the clay minerals (kaolinite, illite, montmorillonite), isoelectric point (iep) of proteins, cation exchange capacity (CEC) of minerals, nature of cation saturating the clays, and temperature (McLaren et aL, 1958; Armstrong & Chesters, 1964; Harter & Stotzky, 1971, 1973;
... A general discussion of the interaction between proteins and solid surfaces can be found in the review by Rabe et al. (2011). Protein adsorption on kaolinite and montmorillonite has been studied by several authors (Violante et al., 1995;Pagel-Wieder et al., 2007;Barral et al., 2008;Fiorito et al., 2008;Helassa et al., 2009Helassa et al., , 2011. ...
... The differences in surface chemistry and surface area fractions are probably more important for the difference in LSZ adsorption than possible LSZ intercalation in Ca-Mont, but the latter cannot be excluded. Intercalation of LSZ in the interlayers of Na-montmorillonite has been reported by Violante et al. (1995) by adsorption experiments and X-ray analysis. In literature a considerably higher adsorption of protein on Mont than on Kao is also observed. ...
Humic acid (HA) as a soil natural organic matter (NOM) can participate in the interaction between proteins and clay minerals, depending on clay type, HA and protein content, and solution conditions. The effect of HA on the interaction of lysozyme (LSZ) with kaolinite (Kao) and montmorillonite (Mont) was investigated at (initial) pHi 5 and 8. In the solutions containing both HA and LSZ, HA/LSZ complexes were formed with a net charge density depending on pH and HA/LSZ mass ratio f. LSZ adsorption on clays in the presence of HA is dominated by adsorption of HA/LSZ complexes. The HA/LSZ mass ratio (fIEP,pH) at the isoelectric point (IEP) is pH dependent. At f < fIEP,pH the HA/LSZ complexes are positively charged and adsorb well to the negatively charged Mont and Kao surface fractions. The adsorption levels on Mont are considerably larger than on Kao, which is mainly due to the much larger area fraction of modestly hydrophobic basal plates of Mont. The presence of HA increased the plateau adsorption of LSZ on Kao and Mont for both pHi values, and the LSZ adsorption increased with increasing HA content and pHi values due to a decreasing mutual repulsion of the bound HA/LSZ complexes. At pHi 8 complications arose for low initial LSZ concentrations, for f < fIEP,pH the HA/LSZ complexes were only weakly positive and formed dispersed aggregates and for f > fIEP,pH the HA/LSZ complexes were negative, both conditions caused relatively high equilibrium concentrations of LSZ in solution that decreased with increasing initial LSZ concentration. The present results enhance our insight in protein soil interactions for the case that clay particles are brought in contact with aqueous solutions that contain modest amounts of both NOM and protein and stress the importance of the NOM/protein mass ratio and solution pH.
... A general discussion of the interaction between proteins and solid surfaces can be found in the review by Rabe et al. (2011). Protein adsorption on kaolinite and montmorillonite has been studied by several authors (Violante et al., 1995;Pagel-Wieder et al., 2007;Barral et al., 2008;Fiorito et al., 2008;Helassa et al., 2009Helassa et al., , 2011. ...
... The differences in surface chemistry and surface area fractions are probably more important for the difference in LSZ adsorption than possible LSZ intercalation in Ca-Mont, but the latter cannot be excluded. Intercalation of LSZ in the interlayers of Na-montmorillonite has been reported by Violante et al. (1995) by adsorption experiments and X-ray analysis. In literature a considerably higher adsorption of protein on Mont than on Kao is also observed. ...
Humic acid (HA) as a soil organic matter (NOM) can participate in the interaction between proteins and clay minerals, depending on clay type, HA and protein content, and solution conditions. The effect of HA on the interaction of lysozyme (LSZ) with kaolinite (Kao) and montmorillonite (Mont) was investigated at (initial) pHi 5 and 8. In the solutions containing both HA and LSZ, HA/LSZ complexes were formed with a net charge density depending on pH and HA/LSZ mass ratio f. LSZ adsorption on clays in the presence of HA is dominated by adsorption of HA/LSZ complexes. The HA/LSZ mass ratio (fIEP,pH) at the iso-electric point (IEP) is pH dependent. At f < fIEP,pH the HA/LSZ complexes are positively charged and adsorb well to the negatively charged Mont and Kao surface fractions. The adsorption levels on Mont are considerably larger than on Kao, which is mainly due to the much larger area fraction of modestly hydrophobic basal plates of Mont. The presence of HA increased the plateau adsorption of LSZ on Kao and Mont for both pHi values, and the LSZ adsorption increased with increasing HA content and pHi values. This result was due to a decreasing lateral repulsion with increasing HA content and pHi. At pHi 8 complications arised for low initial LSZ concentrations, for f < fIEP,pH the HA/LSZ complexes were only weakly positive and formed dispersed aggregates and for f > fIEP,pH the HA/LSZ complexes were negative, both conditions caused relatively high equilibrium concentrations of LSZ in solution that decreased with increasing initial LSZ concentration. The present results enhance our insight in protein soil interactions for the case that clay particles are brought in contact with aqueous solutions that contain modest amounts of both NOM and protein and stress the importance of the NOM/protein mass ratio and solution pH.
Keywords: humic acid, lysozyme (LSZ), clays, pH, mass ratio of HA/LSZ (f), adsorption and removal
... Notably, the authors stressed that OM BE is a substantial component for the formation of organo-mineral associations. In this context, OM BE is considered an agent that both promotes (e.g., Freitas et al., 2015;Sheng et al., 2016) and inhibits aggregation depending on the milieu conditions (e.g., Flynn et al., 2012;Pen et al., 2015), composition, concentration, and its molecular properties (e.g., protein-to-polysaccharide ratio, molecular weight, functional groups, and chemical structure) (e.g., Barreto et al., 2020;Violante et al., 1995;Wang et al., 2012;Zhu et al., 2009). For example, the pH controls the interaction between functional groups of biopolymers that results in internal structural alterations of OM BE (e.g., Becker et al., 2011;Dogsa et al., 2005;Kubiak-Ossowska et al., 2016;Pen et al., 2015;Wang et al., 2012). ...
Background:
Soil organisms influence pedogenesis on a molecular level through the production of biopolymers which interact with soil minerals depending on their molecular properties. Specifically, biopolymers impact structure formation by inhibiting aggregation as a separation agent or promoting aggregation as a bridging agent. Mucus is a biopolymer excreted by earthworms that consists mainly of proteins, polysaccharides, and, to a lesser extent, lipids. However, despite earthworms’ fundamental contribution to soil quality and structuring via bioturbation, the role of mucus in aggregation still has to be unraveled.
Aims:
Our study explores the role of cutaneous earthworm mucus (CEM) of Lumbricus terrestris L. for the formation of organo-mineral associations and aggregates, a sub-process of pedogenesis.
Methods:
We conducted batch experiments with goethite and CEM at different pH values and varying CEM concentrations to form mucus-goethite associations. Employing the newly formed mucus-goethite associations, we explored the (homo-/hetero-) aggregation with quartz particles as a function of the loading of surfaces with mucus-C and CEM concentration in solution.
Results:
Our results suggest that the molecular structure of CEM constituents (especially proteins) is sensitive to pH. We found that the adsorption of CEM to goethite depends on pH and concentration. Polysaccharides of CEM adsorbed preferentially under acidic conditions (pH 3) and at low CEM concentration (6 mg mucus-C L-1). In contrast, stronger adsorption of proteins was observed at higher CEM concentrations (30 mg mucus-C L-1). In subsequent aggregation experiments, the hetero-aggregation rate of organo-mineral associations and quartz decreased at low C-loadings. Conversely, the rate increased at high loadings compared to the CEM-free reference. Furthermore, electrostatic/steric repulsion (separation agent) inhibited the aggregation between goethite particles at high CEM concentrations in the solution (mineral/mucus ratio of 17). However, at a low CEM supply (mineral/mucus ratio of > 83), CEM took the role of a bridging agent.
Conclusions:
The composition, function, and (im-)mobilization of CEM and corresponding organo-mineral associations in earthworm-influenced soil structures are shaped by the structure/reactivity of CEM affected by environmental parameters. Formation and aggregation of mucus-mineral associations contribute to nutrient/carbon storage and are involved in the structure formation in soil.
... Similar results were found by Balme et al. [11]. A ∆ d 001 of~10 Å was reported for other Na + -Mt and explained invoking the intercalation of unfolded lysozyme [40]. The adsorption of lysozyme on other clay minerals (saponite) results in interlayer expansion to d 001 of 44 Å 38 [39]. ...
A synthetic approach to bond lysozyme (LY) to commercial natural carriers, namely clay minerals (bentonite, BN; and sepiolite, SP) and commercial zeolite (Phil 75®, PH), already in use in feed formulation, is proposed. The synthetic route, which implies solid–liquid adsorption, is a simple and effective way for preparing hybrid materials characterized by LY loadings up to 37 mgLY/gcarrier. By operating at pH 4.3, initial LY content of 37.5 mgLY/gcarrier, and reaction time of 90 min, hybrid materials with LY loadings of 37, 35, and 12 mgLY/gcarrier for LY-SP, LY-BN, and LY-PH, respectively were obtained. The LY initial concentration and pH, as well as the physico-chemical properties of the carries were found to be the parameters that govern the synthesis of the materials. The driving force for an effective LY adsorption and interaction is the combined Zero Point Charge (ZPC) of the carriers, always negative (in the range between −4 and −170 mV) and the positive ZPC of LY, as well as the carrier morphology, characterized by mesoporosity (pore dimensions in the range of 5–12 nm). However, it is the interaction of charges of opposite sign that mainly affects LY loadings and bond strength. Based on SEM-EDX analysis, LY molecules are quite homogeneously spread onto the carriers’ surface. TG-DTG analyses showed that the LY–carrier interaction in the hybrid materials is stronger than that in a simple mechanical mixture of the components. Specifically, in the hybrid materials, the phenomenon at 300 °C, associated to LY decomposition, is broadened and slightly shifted towards higher temperatures (320–350 °C), whereas in a mechanical mixture of the same composition, it occurs at temperatures closer to those of free LY, as if there were no or very weak interactions. At pH 3, a very little LY release, 0.03 and 0.01 mgLY/gcarrier, was found for LY-BN and LY-PH, respectively. The latter became larger at pH 7, 0.06 mgLY/gcarrier for both BN and PH carriers, suggesting that BN and PH are better modulators of LY release. The paper provides insights for the study and the development of new optimized feed formulations for the targeted delivery of natural compounds with antimicrobial activity, alternatives to antibiotics, and vaccinal antigens.
... Furthermore, the adsorption of proteins to mineral surfaces is a function of pH, ionic strength, protein concentration, molecular weight, and the type of mineral (Violante et al., 1995;Barreto et al., 2020). It leads to conformation changes of adsorbed proteins which alter their reactivity in the environment. ...
Soil organisms are recognized as ecosystem engineers and key for aggregation in soil due to bioturbation, organic matter (OM) decomposition, and excretion of biogenic OM. The activity of soil organisms is beneficial for soil quality, functions, and nutrient cycling. These attributions are based on field-scale observations that link the presence and activity of organisms to spatiotemporal changes in soil properties and can be traced back to the formation of biogenic aggregates. This biogenic formation pathway encompasses a cascade of processes so far discussed not comprehensively. A more general approach needs to consider the activity and feedback loops between soil biota, the active release of biogenic OM by excretion, the interaction of biogenic OM with soil constituents, the formation of organo-mineral associations, and how these become incorporated in aggregated structures. Especially the function of biogenically excreted OM, which is quite complex in composition, is controversial as it permits or inhibits aggregation. This review analyzes the various roles of biogenically excreted OM may take as an aggregation agent. We will show that its function depends on the interplay of numerous factors, including environmental conditions, variety of OM producers, composition and availability of biogenically excreted OM, and type of interacting mineral phase. We consider biogenically excreted OM to affect aggregate formation in three different ways: (I) as a bridging agent which promotes the aggregation due to surface modifications and attraction, (II) as a separation agent which favors the formation, mobility, and transport of organo-mineral associations and inhibits their further inclusion into aggregates, and (III) as a gluing agent which mediates aggregate stability, after an external force provokes a close approach of soil particles. We conclude that biogenically excreted OM takes these functional roles simultaneously and to a varying extent across spatiotemporal scales. Hence, biogenically excreted OM is involved in the surface modification of soil particles, in the enmeshment and gluing of particles into soil aggregates, in the (im-)mobilization, and in facilitating the transport of particles. All that depends on the interplay of a hierarchy of factors comprising the local soil community's composition, the properties of biogenically excreted OM, and the conditions of the immediate environment.
... In the past, studies on changes in adsorption characteristics before and after the removal of soil and sediment components have focused on the changes in the isotherm adsorption equation or kinetic adsorption equation of each single component 29 The organic/inorganic composite content was not equal to the sum of OM and metal oxides, presumably because hydrogen bonding, ion exchange and hydrophobic forces, such as anion adsorption mechanisms, embedded the composites in the mineral surface and between layers of swollen clay mineral crystals 30,31 , affecting the mineral cementation degree and thus colloid stability 32 . In addition to directly participating in the formation of complexes, the strong surface activity of iron and aluminium oxide can form bridges with OM and stabilize colloids through coordination exchange or the formation of ionic bonds 33 . ...
This study is to determine the spatial distribution characteristics of Cu and Zn adsorption on the sediments of the estuary of Dianchi Lake, as well as the composite adsorption law of Cu and Zn on combinations of sediment organic matter, metal oxides, and organic–inorganic composites. The relationship between the adsorption contribution of each component of the substance. A static adsorption experiment was applied to the sediments in the estuary of Dianchi Lake. The relationship between adsorption capacity and sediment composition was analyzed through correlation analysis and redundant analysis. The results show that along the direction of the river flow and the vertical depth, the adsorption capacity presents a relatively obvious spatial distribution law; the change trend of sediment component content is not the same as the change trend of Cu and Zn adsorption capacity. The change trend of the sediment component content is not the same as the change trend of the adsorption amount of Cu and Zn, and the compound effect between the components affects the adsorption amount. The adsorption of Cu by the four groups of sediments after different treatments is more in line with the Freundlich isotherm adsorption model; When adsorbing Zn, the untreated and removed organic matter and iron-aluminum oxide group are in good agreement with the Freundlich model, while the organic matter-removed group and the iron-aluminum oxide removal group are more consistent with the Langmuir isotherm adsorption model; The adsorption contribution rate of organic–inorganic composites in sediments is not a simple addition of organic matter and iron-aluminum oxides, but a more complex quantitative relationship.
... The surface of sediment particles is not uniform, so the t of the Freundlich isotherm adsorption model aligns with reality; Mustapha 's et al. 24 research results are similar. The order of the adsorption amounts of the samples corresponds to the order of their K values (because some samples could not be tted by the Langmuir isotherm adsorption model, the Freundlich adsorption isotherm model K value was used for comparison), which is similar to most research results 25 The organic/inorganic composite content was not equal to the sum of OM and metal oxides, presumably because hydrogen bonding, ion exchange and hydrophobic forces, such as anion adsorption mechanisms, embedded the composites in the mineral surface and between layers of swollen clay mineral crystals 27,28 , affecting the mineral cementation degree and thus colloid stability 29 . In addition to directly participating in the formation of complexes, the strong surface activity of iron and aluminium oxide can form bridges with OM and stabilize colloids through coordination exchange or the formation of ionic bonds 30 . ...
This study was conducted to determine the spatial distribution characteristics of Cu and Zn adsorption on the sediments of the estuary of Dianchi Lake, as well as the adsorption laws of Cu and Zn on combinations of sediment organic matter, metal oxides, and organic-inorganic composites.A static adsorption experiment was applied to four groups of sediments from the estuary of Dianchi Lake, and results were generated through correlation analysis and redundancy analysis. The four groups were as follows:
(1) Untreated, Group A,
(2) Organic matter removed, Group B,
(3) Iron and aluminium oxide removed, Group C,
(4) Organic matter and iron and aluminium oxide removed, Group D.
The adsorption capacity was correlated with the spatial distribution along the direction of river flow and vertical depth. High contents of various components of the sediment did not correlate with high adsorption capacities for Cu and Zn, according to the use of four groups of sediments subjected to different treatment processes. The adsorption of Cu fit the Freundlich isotherm adsorption model for all four sediment groups. For Zn adsorption, the untreated and removed organic matter and Fe-Al oxide groups were in good agreement with the Freundlich model, while the removed organic matter and removed Fe-Al oxide groups were in good agreement with the Langmuir isothermal adsorption model. The results indicate that there is a quantitative relationship between the adsorption of heavy metals and organic and inorganic complexes in sediments.
... The various types of materials such as biomolecules, carbon-based materials, polymers, etc., have been used in different applications such as adsorption, catalyst, sensors, etc. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15]. The interaction of any adsorbent molecules of biomolecules has intensively worked in the past [16][17][18][19][20][21][22][23][24]. Besides, the bovine serum albumin (BSA) has spherical dimensions of about 4-14 nm [25]. ...
In this research, the adsorption of bovine serum albümin (BSA) onto activated carbon (AC) obtained from apple bark was carried out and the thermodynamic parameters of adsorption process were investigated. Besides, the functions involved in BSA attachment were examined by adsorption experiments on retention capacities for BSA at 298 K, pH of 7, ionic strength of 5.10−2 M, and initial concentration of 5.10−2 g L−1, respectively. The bovine serum albumin (BSA) adsorption experiment onto activated carbon (AC) indicated that the highest adsorption yield was achieved at pH 5.5. The BSA molecules at pH 5.5 are very stable and that pH value is close to isoelectronic point of BSA. The surface structural change of BSA and activated carbon was studied before and after the experiment using scanning electron microscopy (SEM) analysis and Fourier Transform Infrared Spectroscopy (FTIR). By the way, the thermodynamic functions such as Gibbs free energy (ΔG∗), activation energy (Ea), activation entalphy (ΔH∗), and activation entropy (ΔS∗) were calculated as − 66.17, 37.73, − 29.09 kJ mol−1, and + 124.42 J mol−1 K−1 for bovine serum albumin (BSA) adsorption, respectively. The adsorption of the process was investigated using Eyring and Arrhenius equations and the adsorption kinetic of BSA on AC was found to be coherent with the pseudo-second-order model.
Graphical Abstract
In order to develop new effective medical sorbents, the sorption ability of synthetic montmorillonite of various compositions in relation to model protein molecules - oxytocin, albumin and immunoglobulin in a synthetic biological fluid (pH=7.2) was studied. Montmorillonites with varying degrees of isomorphic substitution of aluminum atoms for magnesium atoms in octahedral layers were synthesized under hydrothermal conditions (Na2x(Al2(1-x),Mg2x)Si4O10(OH)2.nH2O, where x=0.5, 0.9 and 1). According to the SEM data, the samples had a layered morphology, forming spongy aggregates with secondary pores with a diameter of up to 300-350 nm. To model the kinetics of protein adsorption on montmorillonites, pseudo-first (PFO) and pseudo-second order (PSO) equations were used, and Langmuir, Freundlich and Temkin models were used to process isotherms. It has been established that the sorption capacity of synthetic montmorillonite in relation to proteins can be 2-4 times higher than the capacity of raw clays and can reach 90, 250 and 900 mg/g for oxytocin, albumin and immunoglobulin, respectively. The presence of competitive adsorption between proteins and components of simulated body fluid has been discovered. The dependence of the adsorption capacity of the samples in relation to proteins of different molecular weights on the chemical composition of montmorillonite and the degree of isomorphic substitution of magnesium with aluminum in the octahedral layers has been established. The possibility of developing sorbents for the extraction of proteins from complex mixtures based on synthetic combined montmorillonites has been demonstrated.
2-Methylisoborneol (2-MIB), a major cause of taste and odor (T&O) in drinking water, tends to combine with the coexisting dissolved organic matter (DOM), resulting in the altering of the physico-chemical properties of 2-MIB, and its corresponding migration and removal behaviors during water treatment. In this study, dialysis and ultrafiltration were set up to differentiate bound and free 2-MIB, and the binding of different types of DOMs to 2-MIB was investigated. At 298 K, the Freundlich constants, KF ((ng/mg)(L/ng)1/n), representing the combination ability between 2-MIB and the model pollutants, followed the order hen egg-white lysozyme (HEL 22.52) > bovine serum albumin (BSA 19.23) > sodium carboxymethyl cellulose (CMC 9.69) > sodium alginate (SA 7.78), and humic substances exhibited a weak association with 2-MIB, indicating that aromatic structures provide few binding sites. The KF of natural organic matter (NOM 196.15) in real raw water was much larger than that of model pollutants owing to the complex composition and the wide molecular weight distribution. The major interactions between DOM and 2-MIB were van der Waals forces and hydrogen bonding, except that HEL were hydrophobic interactions. Common metal ions affected these associations and generally decreased KF. The findings in this study are of particular significance for the selection of proper strategy to maintain the water quality during the outbreak of T&O in the source.
This paper concisely reviews the general principles underlying protein adsorption from aqueous solution onto a solid surface. The discussion includes the various stages of the adsorption process, i.e., transport of the protein molecules towards the surface, the absorbed amount under equilibrium conditions, desorption and readsorption. Among the interactions that determine the overall protein adsorption process (1) redistribution of charged groups in the interfacial layer, (2) changes in the hydration of the sorbent and the protein surface, and (3) structural rearrangements in the protein molecule play major roles. Special attention is given to the relation between the structural stability of the protein molecule and its adsorption behaviour.
A theoretical basis is developed for the classification of adsorption isotherms for solutes in dilute solution, which relates their characteristic shapes to parameters of the solvent and any second solute.Three of the four main classes (S, L, H) are accounted for by differences in relative magnitude of the activation energies of desorption of solutes and solvent. The S-shaped isotherm is also accounted for by an additional concentration-dependence of this parameter, implying cooperative adsorption. The subgroups of each class are also explained.The linear (C) isotherm is explained by penetration of substrate micropores by solute, with or without solvent, whereby new adsorption sites are opened up; the theory predicts the experimentally found sharp inflection to a plateau in the C-curves. Curves of the other classes can also in some cases show a linear branch above the turning point, representing conditions like those for C-curves.The theoretical treatment is valid whether the adsorbing surface is energetically uniform, or, as in most systems, nonuniform.
X‐ray diffractometry, electron microscopy, electrophoretic mobility, and pH measurements have been used in an attempt to understand the mechanisms involved in the formation of stable clay‐protein complexes. Pepsin was apparently adsorbed to positive edge sites on the smectite, and all other proteins were adsorbed on planar surfaces. Casein, chymotrypsin, lysozyme, and ovomucoid intercalated the H and Na‐smectites, and catalase may have intercalated the H‐smectite. All proteins, except catalase, appeared to intercalate the clays when the weight ratio of protein‐to‐clay exceeded about 1:5. Catalase did not appear to intercalate the Ca, Al, La, or Th‐smectites, even though the ratio of adsorbed protein‐to‐clay exceeded 1:5. The electrophoretic mobility of the H, Na, and Th‐smectites became less negative upon adsorption of proteins, indicating a physical or chemical covering of the negative clay surface charge and a flocculation of the complex. When protein was adsorbed by Ca, Al, and La‐smectites they either showed little change or increased in electrophoretic mobility, indicating a tendency for clay tactoids to be broken. The pH of all protein‐clay complexes tended to approach neutrality. In the acid clays (H, Al, and Th‐smectite), especially, this indicated a cation exchange reaction, since the exchanged inorganic ions would be subsequently removed from the clay.
Two proteins, lysozyme and ovalbumin, were adsorbed to smectite homoionic to first five elements of the alkali and the alkali earth series, hydrogen, aluminum, and lanthanum. Lysozyme was also adsorbed to Na-smectite in the presence of 10−3to 2.0 N NaCl solutions. Lysozyme adsorption by all clays was of the high affinity type, with 93 percent of the variance in the adsorption maximum being accounted for by the equation Amax= 290.2 – 5.5 (valence) – 96.8 (ionic radius) – 114.3 (Pauling electronegativity) – 6.5 (ionization potential). Adsorption of lysozyme by Na-smectite was decreased when the reaction occurred in solution with NaCl concentrations of 0.5 N or above. Adsorption of ovalbumin by the smectites was of the constant partition type and pH dependent. All homoionic clays except those saturated with H and Be adsorbed similar amounts of ovalbumin. Attempts to desorb the two proteins were generally unsuccessful, indicating that stabile complexes had been formed.
The adsorption of urease on a montmorillonite (M), a non-crystalline aluminium hydroxide (AL) and an Al(OH)x-montmorillonite complex (AM) as well as the activity, the kinetics and the stability of the enzyme-clay mineral complexes were studied. The equilibrium adsorption isotherms of urease on clay minerals fitted both the Langmuir or the Freundlich equations. The Langmuir adsorption isotherm of the enzyme on M was of H type (“high affinity”) whereas the isotherms on AL and AM were of L type (“Langmuir”). On adding up to 21.2mg of enzyme g−1 clay, the amount of urease held on the clay minerals followed the order M > AM > AL throughout the pH range explored (4.0–9.0). The adsorption of urease on M, AM and AL was differently affected by pH. The specific activity of enzyme immobilized on M and AM was relatively high (71 and 64% respectively) as compared to that of the free enzyme; in contrast the specific activity of urease adsorbed on AL was considerably reduced (15%). The free and immobilized urease showed similar pH- and temperature-activity profiles and both states obeyed Michaelis-Menten kinetics. The Vmax and Km parameters, as well as the thermal stability of adsorbed urease were always lower than those of the free urease, whereas the proteolytic stability of urease held on AL was higher than that of the enzyme free or adsorbed on M and AM. Finally, it was ascertained that the covering of the surfaces of montmorillonite with different amounts of OH-A1 species reduced the quantity as well as the activity of adsorbed enzyme.
Hen egg-white lysozyme (isoelectric point near pH 11) has been adsorbed at 20°C on synthetic hydroxyapatites and fluorapatite, and on a commercial hydroxyapatite used as chromatography support. The influences of pH (5.5–8), ionic strength, addition of calcium and phosphate ions to the solution have been examined and related to the electrophoretic mobility of the adsorbent. Adsorption was rapid and reversible with respect to dilution. The maximum amount adsorbed was in the range 0–55 nmol M−2 depending on the adsorbent and experimental conditions, the highest value being typical of a loosely packed monolayer. Adsorption occurred only when the surface carried an electric charge opposite to that of the protein. The dependence of adsorption on the solution composition demonstrated that it was driven by electrostatic interactions. Electrophoretic mobility measurements in the presence of lysozyme confirmed that the latter ensures surface charge neutralization. Accordingly, a correlation has been found between the maximum amount adsorbed and the apparent zeta potential of the adsorbent at zero coverage. The variation of the affinity constant deduced from adsorption isotherms indicated that the potential acting on the molecules adsorbed at low coverage is close to the apparent zeta potential of the adsorbent. A formulation of the adsorption isotherm derived from the Langmuir equation has been worked out in order to account for the progressive decrease of the interaction potential with increasing coverage