Colloidal stability of dextran-modified latex particles toward adsorption of concanavalin A

The colloidal stability of the dextran-modified poly(methyl methacrylate) (PMMA) latex particles toward adsorption of a carbohydrate-binding protein, concanavalin A (Con A), is primarily controlled by the charge neutralization mechanism. Formation of a crosslinked network structure via the specific affinity interactions between the dimeric Con A molecules and the dextran molecules anchored onto different latex particles may also have an impact on the coagulation kinetics. Judging from the data of coagulation kinetics, the colloidal stability of the latex particles toward added Con A in the decreasing order is: latex particles without dextran modification>latex particles with a dextran content of 2.15%>latex particles with a dex-tran content of 1.24% based on total polymer weight (PMMA+grafted dextran). The coagulation mechanisms involved in the adsorption of Con A onto the latex particles have been proposed to explain these experimental data. Charge neutralization of the negatively charged latex particles by adsorption of the positively charged Con A is the predominant destabilization mechanism. The ratio of the number of dextran active sites to that of Con A molecules plays an important role in the formation of the crosslinked network structure. The electrolytes in water cause a reduction in the electrostatic repulsion force among the interactive latex particles, but this ionic strength effect is not significant in comparison with charge neutralization. Received: 22 October 1997 Accepted: 24 December 1997     

Isothermal equilibrium adsorption of concanavalin A on dextran-modified poly(methyl methacrylate) latex particles

Isothermal equilibrium adsorption experiments were carried out to study the adsorption of concanavalin A (Con A) on dextran-modified poly(methyl methacrylate) (PMMA) latex particles. Three PMMA particles with various levels of dextran modification were selected for study: 0% (designated as D0), 1.24% (D20), and 2.45% (D75) based on total polymer weight. The Langmuir model is applicable to both D0 and D20 systems, although the data for the D20 system are somewhat scattered. On the other hand, the amount of Con A adsorbed per gram polymer particles (q*) versus the Con A concentration in water (c*) curve for the D75 system cannot be described by the Langmuir model. The deviation is attributed to the formation of a crosslinked network structure, caused by specific binding of the dimeric Con A molecules onto two neighboring particles with grafted dextran. The ratio of the initial number of Con A molecules to the initial number of active binding sites on the dextran-modified particle surface plays an important role in determining the structure of flocs formed. The maximum amount of Con A adsorbed on the particle surface (q _max) is of the order of 10⁻¹ μmol per gram particles and q _max in decreasing order is D75 > D20 > D0. The dissociation constant of the Con A-D20 (or Con A-D75) pair is of the order of 10⁻¹ μmol dm⁻³ which is 1 order of magnitude smaller than that of the Con A-D0 pair. Thus, the electrostatic interaction between Con A and D0 is much weaker than the affinity interaction between Con A and D20 (or D75). An empirical model is proposed to qualitatively explain the q* versus c* data. Received: 18 June 1998 Accepted in revised form: 24 December 1998     

Adsorption of different amphiphilic molecules onto polystyrene latices

In order to know the influence of the surface characteristics and the chain properties on the adsorption of amphiphilic molecules onto polystyrene latex, a set of experiments to study the adsorption of ionic surfactants, nonionic surfactants and an amphiphilic synthetic peptide on different latex dispersions was performed. The adsorbed amount versus the equilibrium surfactant concentration was determined. The main adsorption mechanism was the hydrophobic attraction between the nonpolar tail of the molecule and the hydrophobic regions of the latex surface. This attraction overcame the electrostatic repulsion between chains and latex surface with identical charge sign. However, the electrostatic interactions chain-surface and chain-chain also played a role. General patterns for the adsorption of ionic chains on charged latex surfaces could be established. Regarding the shape, the isotherms presented different plateaus corresponding to electrostatic effects and conformational changes. The surfactant size also affects the adsorption results: the higher the hydrophilic moiety in the surfactant molecule the lower the adsorbed amount.     

Agglutination study of poly(allyl-alpha-D-glucopyranose/styrene) latex particles in the presence of Concanavalin A

Latex agglutination of the glucose-modified latex, which were synthesized by emulsion copolymerization of allyl-modified glucose and styrene monomers by specific binding interactions between Concanavalin A (Con A) and allyl-alpha-d-glucopyranose, were investigated. The surface of the glucose-modified latex was characterized by dye-partition method and the number of glucose was 1517 per latex particle. The average particle size and the polydispersity index of the latex were 78.3 and 1.005, respectively. Time-evolution adsorption behavior of various concentrations of the Con A and the consequent latex agglutination were studied by UV spectrophotometer at 540 nm and zeta-potential analyzer at the fixed latex concentration of 0.02 wt%. Specific binding between Con A and allyl-alpha-D-glucopyranose leaded the latex particles to coagulate by decreasing the electrostatic repulsion between the particles and mobility.     

Concanavalin a immobilized affinity adsorbents for reversible use in yeast invertase adsorption

Concanavalin A (Con A) immobilized poly(2-hydroxyethyl methacrylate) (PHEMA) beads were investigated for specific adsorption of yeast invertase from aqueous solutions. PHEMA beads were prepared by a suspension polymerization technique with an average size of 150-200 microm, and activated by epichlorohydrin. Con A was then immobilized by covalent binding onto these beads. The maximum Con A immobilization was found to be 10 mg/g. The invertase-loading capability of the PHEMA/Con A beads was 107 mg/g. The maximum invertase adsorption capacity on the PHEMA/Con A adsorbents was observed at pH 5.0. The values of the Michaelis constant K(m) of invertase were significantly larger upon adsorption, indicating decreased affinity by the enzyme for its substrate, whereas V(max) was smaller for the adsorbed invertase. Adsorption improved the pH stability of the enzyme as well as its temperature stability. Thermal stability was found to increase with adsorption. The adsorbed enzyme activity was found to be quite stable in repeated experiments. Storage stability of adsorbed invertase.     

Dextran stabilized poly(methyl methacrylate) latex particles and their potential application for affinity purification of lectins

 Stable poly(methyl methacrylate) latex particles (220–360 nm in diameter) using dextran as the protective colloid were prepared and characterized in this study. Such an emulsion polymerization system follows Smith–Ewart Case III kinetics (i.e., the average number of free radicals per particle is greater than 0.5) due to the relatively large latex particle size. The carbohydrate content of these dextran modified emulsion polymers shows a maximum around 5% dextran based on total monomer weight. The latex stability during polymerization is closely related to the carbohydrate content of these latex particles, and it is controlled by the ratio of the thickness of the dextran adsorption layer to that of the electric double layer of the latex particles. The critical flocculation concentration (CFC) of the latex products correlates well with the latex stability during polymerization. The greater the total scrap produced during polymerization (i.e., the stronger the bridging flocculation), the lower is the CFC of the latex products. The affinity precipitation of concanavalin A (a model lectin used in this study) by the dextran modified PMMA latex particles is also illustrated in this study. The specifically adsorbed concanavalin A increases with the carbohydrate content of the dextran modified latex particles. Received: 9 December 1996 Accepted: 8 April 1997     

Effects of Cosolvents and pH on Protein Adsorption on Polystyrene Latex: A Dynamic Light Scattering Study

Dynamic light scattering was used to study the adsorption of two proteins with different surface properties (IgG and HSA) on negatively charged polystyrene latex. The proteins were adsorbed from water and from water/methanol and water/glycerol mixtures at various pH. Some striking differences between the adsorption behaviors of the proteins were observed. Whereas the thickness of the adsorbed layer of HSA was extremely sensitive to pH and solvent composition, that of IgG was not. IgG mainly showed an end-on orientation on polystyrene whereas several different surface orientations are suggested for HSA under different conditions. The addition of methanol inhibited the adsorption of HSA on the latex, but it did not affect the adsorption of IgG. In contrast, the addition of glycerol increased the thickness of the adsorbed layers of both proteins. So, the orientation of IgG on the latex is insensitive to pH but is a function of the kind of solvent whereas both pH and solvent strongly affect the adsorption of HSA. This is a puzzling result since both cosolvents should equally affect the adsorption of both proteins if the dominant forces for adsorption are the same. Therefore, we concluded that, whereas hydrophobic interactions are the dominant force in the adsorption behavior of HSA, van der Waals forces are the main forces involved in the attachment of IgG to the lattices. Copyright 2000 Academic Press.     

Charge regulation enables anionic hydroxypropyl guar-borate adsorption onto anionic and cationic polystyrene latex

Reported are adsorption isotherms for guar and hydroxypropyl guar (HPG), with and without the presence of borate ions, onto surfactant free anionic polystyrene latex. Guar and HPG formed adsorbed monolayers on the hydrophobic latex. The presence of borate ions converted the nonionic guar and HPG into an anionic polyelectrolyte. However, there was no measurable influence of bound borate ions on the adsorption of guar or HPG onto anionic, hydrophobic latex. To underscore the unusual behavior of HPG-borate, a sample of HPG was oxidized to introduce carboxyl groups, and the adsorption of the carboxylated HPG onto anionic polystyrene was measured. Unlike HPG-borate, oxidized HPG did not adsorb onto negative polystyrene latex at neutral pH because of electrostatic repulsion. To explain the adsorption of negative HPG-borate onto negative latex, we proposed that as HPG-borate segments approach the latex surface, the negative electrostatic potential near the latex surface induces the detachment of the labile borate groups from HPG.     

Electrostatic interactions between amphoteric latex particles and proteins

The electrostatic interactions between amphoteric polymethyl methacrylate latex particles and proteins with different pI values were investigated. These latex particles possess a net positive charge at low pH, but they become negatively charged at high pH. The nature and degree of interactions between these polymer particles and proteins are primarily controlled by the electrostatic characteristics of the particles and proteins under the experimental conditions. The self-promoting adsorption process from the charge neutralization of latex particles by the proteins, which have the opposite net charge to that of the particles, leads to a rapid reduction in the zeta potential of the particles (in other words colloidal stability), and so strong flocculation occurs. On the other hand, the electrostatic repulsion forces between similarly charged latex particles and the proteins retard the adsorption of protein molecules onto the surfaces of the particles. Therefore, latex particles exhibit excellent colloidal stability over a wide range of protein concentrations. A transition from net negative charge to net positive charge, and vice versa (charge reversal), was observed when the particle surface charge density was not high enough to be predominant in the protein adsorption process.     

12-Mercaptododecyl β-maltoside-modified gold nanoparticles: specific ligands for concanavalin A having long flexible hydrocarbon chains

A simple and highly specific protein detection system using glycoconjugated gold nanoparticles was investigated. This system was based on the aggregation of gold nanoparticles coated with carbohydrate alkanethiols in the presence of corresponding proteins (lectins) that had specific recognition for certain carbohydrates. In order to construct an efficient specific recognition system, maltoside alkanethiol was adopted as an effective sensing modifier having a disaccharide group and a flexible long alkyl chain. The surface modification of gold nanoparticles with maltoside alkanethiol resulted in a shift and broadening (from 520 to 610 nm) of the absorption peak. Monodispersed maltoside-adsorbed gold nanoparticles aggregated with the specific lectin, concanavalin A (Con A). This phenomenon was used to detect the presence of Con A and to estimate concentrations of Con A in sample solutions. The precipitate of the maltoside–gold nanoparticle–Con A mixture was redispersed by addition of methyl α-D-mannopyranoside whose adsorption coefficient is larger than that of maltoside with Con A.     

Protein Adsorption on a Glycosylated Polyacrylonitrile Surface: Monitoring with QCM and SPR

A simple and efficient method to fabricate a glycosylated surface on a polyacrylonitrile‐based film is described. Construction and protein adsorption processes were monitored in situ using a QCM. A PANCHEMA film was deposited on the gold surface of the quartz crystal, and the glycosylated surface was then constructed through surface modification. Con A and BSA were used as probes to study the specificity of this surface to proteins. It can recognize Con A, while showing no specific interaction with BSA. The binding affinity indicates the presence of strong multivalent interactions between Con A and the glucose residues (cluster glycoside effect). Reproducibility and repeatability of the glycosylated polymer surface are sufficient to allow specific adsorption of Con A.

     

Participation of electrolyte cations in albumin adsorption onto negatively charged polymer latices

The participation of electrolyte cations in the adsorption of bovine serum albumin (BSA) onto polymer latices was investigated. The latices used were hydrophobic polystyrene (PS), and hydrophilic copolymers, i.e., styrene (St)/2-hydroxyethyl methacrylate(HEMA) copolymer [P(St/HEMA)] and styrene/acrylamide (AAm) copolymer [P(St/AAm)]. Three kinds of electrolyte cations (Na⁺, Ca²⁺, Mg²⁺) were used as the chloride. The amount of BSA adsorbed in every cation medium showed a maximum near the isoelectric point (iep, pH about 5) of the protein. The amounts of BSA adsorbed onto copolymer latices (except in the acidic pH region lower than the iep) were considerably smaller than that onto PS latex because of the steric repulsion and the decrease in the hydrophobic interaction between BSA and copolymer latices. In the acidic pH region, there was little difference in the amount of BSA adsorbed in every cation medium. However, in the pH region higher than the iep, the amounts of BSA adsorbed (particularly onto PS latex) in divalent cations (Ca²⁺ and Mg²⁺) media were relatively greater compared with that in a monovalent (Na⁺) one. This result was interpreted on the basis of the differences in such effects of electrolyte cations as dehydration power, suppression of the electrostatic repulsion, and binding affinity to BSA molecule. Ion Chromatographic estimation of the amounts of electrolyte cations captured upon BSA adsorption (in pH > 5) revealed that divalent cations were incorporated into the contact interface between the latex and BSA molecule so as to prevent the accumulation of anion charge and facilitate the protein adsorption.     

Adsorption behavior of methylene blue and its congeners on a stainless steel surface

Methylene blue and its congeners as model dyes were adsorbed onto stainless steel particles at different ionic strengths, pH values, and ethanol contents, and the adsorption mechanism was investigated. A Fourier transform infrared spectroscopy (FTIR) analysis of the dyes adsorbed on the stainless steel plate was carried out to determine the orientations of the adsorbed dyes on stainless steel surface. The adsorption isotherms for all the dyes tested were approximated by a Langmuir equation (Q=Kq(m)C/(1+KC)) in most cases except under strongly basic conditions. From the ionic strength and ethanol content dependencies of the K value in the Langmuir equations, both the electrostatic and hydrophobic interactions were indicated to contribute to the adsorption of the dyes at neutral pH. By comparing the K and q(m) values for the methylene blue congeners and with the aid of the FTIR analyses, it was found that the kind of substituent groups at most positions of the polyheterocycles of methylene blue strongly affects the adsorption behavior, particularly the area occupied by an adsorbed dye molecule, the affinity for the stainless steel surface, and the orientation of the adsorbed dye molecule on the stainless steel surface.     

Effects of oligonucleotide adsorption on the physicochemical characteristics of a nanoparticle-based model delivery system for antisense drugs

Cationic polystyrene nanoparticles, as a model drug carrier system for nucleic acids, are capable of binding negatively charged oligonucleotides by multiple electrostatic interactions. The effect of the adsorption of phosphorothioate oligonucleotides on the physicochemical properties of the carrier system was investigated for uncoated and sterically stabilized latex particles. Turbidity measurements and photon-correlation spectroscopy indicate that the colloidal stability of the nanoparticle-oligonucleotide conjugates is influenced by the number of oligonucleotides adsorbed on the carrier. Especially in the case of the uncoated material, a destabilizing effect has been observed up to oligonucleotide concentrations of 2.7 μmol/g polymer. Strikingly, at higher concentrations the latexes exhibit colloidal stability similar to the oligonucleotide-free samples. These results were correlated to zeta-potential measurements demonstrating a reversal from positive to negative values of the zeta potential with increasing oligonucleotide concentration. The points of zero charge of the particles are in the region of maximum coagulation. These findings were compared to adsorption studies and calculations based on the random sequential adsorption model. It appears that at first the colloidal stability of the carrier systems is diminished with increasing oligonucleotide adsorption, while higher surface coverages lead to a significant reduction in coagulation. At the saturation level the surface coverage can be considered as a monolayer of “side-on” adsorbed molecules and the conjugates exhibit colloidal stability similar to the bare particles without adsorbed molecules. Received: 20 April 1998 Accepted: 16 July 1998     

Controlled adsorption-desorption of cationic polymers on the surface of anionic latex particles

The complexation of anionic latex particles with two series of cationic copolymers is studied. The copolymers of the first series contain cationic and electroneutral (zwitter ion) hydrophilic units. The electrostatic adsorption of these copolymers on the surface of latex particles is accompanied by the formation of multiple salt bridges between cationic copolymer units and surface anionic groups. The dependence of ultimate adsorption on the molar fraction of cationic groups in copolymer α is described by a bell-shaped curve with a maximum at α = 0.05−0.10 and a long horizontal portion at α > 0.24. In terms of the adsorption theory of polyampholytes, such a pattern of the adsorption curve results from the compromise between the attraction of polymer chains to the surface induced by their polarization in the electric field of particles and the repulsion of like charged macromolecular units. The stability of complexes with the copolymers of the first series in water-salt media increases with an increase in α. The copolymers of the second series contain cationic and hydrophobic units. In this case, an increase in α is accompanied by a decrease in the amount of the adsorbed polymer throughout the studied α range (0.24–1). The complexes are stabilized not only via electrostatic interactions but also via hydrophobic interactions. A decrease in α decreases the role of electrostatics in stabilization of the complexes; however, this effect is compensated for by an increase in the number of hydrophobic contacts. This allows the stability of complexes to be preserved in concentrated water-salt solutions. The results of this study indicate that the stability of interfacial layers with the participation of cationic copolymers can be changed in a wide range by varying the ratio of ionic and electroneutral (hydrophilic or hydrophobic) comonomers in macromolecules.     

Electrochemical determination of sugars by use of multilayer thin films of ferrocene-appended glycogen and concanavalin A

Multilayer thin films containing concanavalin A (Con A) and ferrocene-appended glycogen (FcGly) were prepared by a layer-by-layer deposition Con A and FcGly by biological affinity (lectin–sugar interaction) on a glassy-carbon electrode. The electrochemical response of the Con A–FcGly film-coated electrode to sugars was investigated. A cyclic voltammogram (CV), typical of redox species confined to the surface of the electrode, was obtained. The peak current (resulting from the electric charge involved in the redox reaction) in the CV from the electrode decreased on addition of sugars in the solution, because the amount of FcGly on the electrode surface decreased as a result of disintegration of the Con A–FcGly film on addition of sugar. Thus, d-glucose and other sugars at millimole per liter levels can be detected by use of Con A–FcGly films-coated electrodes.     

Effect of polyethylene oxide on the viscosity of dispersions of charged silica particles: interplay between rheology, adsorption, and surface charge

In this study a systematic investigation on the adsorption of polyethylene oxide (PEO) onto the surface of silica particles and the viscosity behavior of concentrated dispersions of silica particles with adsorbed PEO has been performed. The variation of shear viscosity with the adsorbed layer density, concentration of free polymer in the solution (depletion forces), polymer molecular weight, and adsorbed layer thickness at different salt concentrations (range of the electrostatic repulsion between particles) is presented and discussed. Adsorption and rheological studies were performed on suspensions of silica particles dispersed in solutions of 10⁻² M and 10⁻⁴ M NaNO₃ containing PEO of molecular weights 7,500 and 18,500 of different concentrations. Adsorption measurements gave evidence of a primary plateau in the adsorption density of 7,500 MW PEO at an electrolyte concentration of 10⁻² M NaNO₃. Results indicate that the range of the electrostatic repulsion between the suspended particles affects both adsorption density of the polymer onto the surface of the particles and the viscosity behavior of the system. The adsorbed layer thickness was estimated from the values of zeta potential in the presence and absence of the polymer and was found to decrease with decreasing the range of the electrostatic repulsive forces between the particles. Experimental results show that even though there is a direct relation between the viscosity of the suspension and the adsorption density of the polymer onto the surface of the particles, variation of viscosity with adsorption density, equilibrium concentration of the polymer, and range of the electrostatic repulsion cannot be explained just in term of the effective volume fraction of the particles and needs to be further investigated. Received: 15 February 2000/Accepted: 26 June 2000     

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.     

Eluotropic strength in non-aqueous liquid chromatography with porous graphitic carbon

Protein adsorption can be either endothermic or exothermic depending upon the protein, the sorbent and process conditions. In the case of protein adsorption onto ion-exchange surfaces exothermic adsorption heats are usually characterized as representing the electrostatic interaction between two oppositely charged surfaces. Endothermic adsorption heats are typically characterized as representing protein reconfiguration and/or repulsive interactions between adsorbed molecules. In certain segments of the literature surface dehydration and solution non-idealities have been suggested as possible sources of endothermic heats of adsorption. Each of these phenomena was investigated during studies concerning the adsorption of bovine serum albumin and ovalbumin onto an anion-exchange sorbent. The results demonstrated that electrostatic repulsive interactions between adsorbed molecules appears to be a larger contributor to endothermic heats of adsorption than surface dehydration or solution non-idealities. The presence of mobile phase cations can reduce the magnitude of endothermic adsorption heats by screening repulsive interactions between adsorbed molecules. Although water release was not found to be a major contributor to endothermic adsorption heats, it is likely to be a contributor to the entropic driving force associated with the adsorption of bovine serum albumin.     

A physico-chemical investigation of poly(ethylene oxide)-block-poly(L-lysine) copolymer adsorption onto silica nanoparticles

The adsorption behavior of poly(ethylene oxide)-b-poly(L-lysine) (PEO(113)-b-PLL(10)) copolymer onto silica nanoparticles was investigated in phosphate buffer at pH 7.4 by means of dynamic light scattering, zeta potential, adsorption isotherms and microcalorimetry measurements. Both blocks have an affinity for the silica surface through hydrogen bonding (PEO and PLL) or electrostatic interactions (PLL). Competitive adsorption experiments from a mixture of PEO and PLL homopolymers evidenced greater interactions of PLL with silica while displacement experiments even revealed that free PLL chains could desorb PEO chains from the particle surface. This allowed us to better understand the adsorption mechanism of PEO-b-PLL copolymer at the silica surface. At low surface coverage, both blocks adsorbed in flat conformation leading to the flocculation of the particles as neither steric nor electrostatic forces could take place at the silica surface. The addition of a large excess of copolymer favoured the dispersion of flocs according to a presumed mechanism where PLL blocks of incoming copolymer chains preferentially adsorbed to the surface by displacing already adsorbed PEO blocks. The gradual addition of silica particles to an excess of PEO-b-PLL copolymer solution was the preferred method for particle coating as it favoured equilibrium conditions where the copolymer formed an anchor-buoy (PLL-PEO) structure with stabilizing properties at the silica-water interface.