Intermolecular interactions in the bilirubin-cholate-silica system

Bilirubin-cholate interactions in aqueous solutions were studied. The constants of binding of bilirubin with taurocholate dimers and taurodeoxycholate trimers were calculated. The adsorption of bilirubin and cholates on the surface of highly dispersed silica was studied. It was shown that taurine-conjugated cholates are poorly adsorbed from micellar solutions on the silica surface, the specific amount of bilirubin adsorbed decreases with increasing concentration of cholates in the solution, the affinity of free bilirubin for the silica surface is independent of the nature of the cholic acid, and that the affinity of cholate-bilirubin complexes for the silica surface is lower than the affinity of free bilirubin.     

Molecular packing of lysozyme,fibrinogen, and bovine serum albumin on hydrophilic and hydrophobic surfaces studied by infrared-visible sum frequency generation and fluorescence microscopy

Infrared-visible sum frequency generation (SFG) vibrational spectroscopy, in combination with fluorescence microscopy, was employed to investigate the surface structure of lysozyme, fibrinogen, and bovine serum albumin (BSA) adsorbed on hydrophilic silica and hydrophobic polystyrene as a function of protein concentration. Fluorescence microscopy shows that the relative amounts of protein adsorbed on hydrophilic and hydrophobic surfaces increase in proportion with the concentration of protein solutions. For a given bulk protein concentration, a larger amount of protein is adsorbed on hydrophobic polystyrene surfaces compared to hydrophilic silica surfaces. While lysozyme molecules adsorbed on silica surfaces yield relatively similar SFG spectra, regardless of the surface concentration, SFG spectra of fibrinogen and BSA adsorbed on silica surfaces exhibit concentration-dependent signal intensities and peak shapes. Quantitative SFG data analysis reveals that methyl groups in lysozyme adsorbed on hydrophilic surfaces show a concentration-independent orientation. However, methyl groups in BSA and fibrinogen become less tilted with respect to the surface normal with increasing protein concentration at the surface. On hydrophobic polystyrene surfaces, all proteins yield similar SFG spectra, which are different from those on hydrophilic surfaces. Although more protein molecules are present on hydrophobic surfaces, lower SFG signal intensity is observed, indicating that methyl groups in adsorbed proteins are more randomly oriented as compared to those on hydrophilic surfaces. SFG data also shows that the orientation and ordering of phenyl rings in the polystyrene surface is affected by protein adsorption, depending on the amount and type of proteins.     

Thermal Stability of Silica-polyorganosiloxane Systems

Thermal stability of silica-polyvinylsiloxane systems with different ratio of hydrophilic and hydrophobic components and its forms with adsorbed dyes were studied using thermochemical method. It was shown that the maximum of endoeffect relating to dehydroxylation of sorbents surface is observed at 325°C. The maximum temperature of the second endoeffect depends on the organosilica sorbents composition and increases with a decrease of hydrophilic and hydrophobic sites ratio. It was found that the values of the summary thermal effect lowers upon the raise of the quantity of silanol groups. It was shown that thermal stability of composition materials is higher in comparison with initial organosilica sorbents. This revised version was published online in August 2006 with corrections to the Cover Date.     

Micellization of bile salts in a formamide solution: A gas liquid chromatography study

Sodium cholate and sodium deoxycholate dissolved in formamide were applied as stationary phases in gas chromatography. The critical micelle concentration of sodium cholate and deoxycholate in formamide was determined by surface tension measurements. The relation of retention times vs. concentration of bile salts was investigated for isomers of monoterpenes and xylenes. The enthalpy of binding of selected compounds with sodium cholate and sodium deoxycholate monomers and micelles was determined.     

Adsorption characteristics of zwitterionic diblock copolymers at the silica/aqueous solution interface

The adsorption of a zwitterionic diblock copolymer, poly(2-(diethylamino)ethyl methacrylate)-block-poly(methacrylic acid) (PDEA59-PMAA50), at the silica/aqueous solution interface has been characterised as a function of pH. In acidic solution, this copolymer forms core-shell micelles with the neutral PMAA chains being located in the hydrophobic cores and the protonated PDEA chains forming the cationic micelle coronas. In alkaline solution, the copolymer forms the analogous inverted micelles with anionic PMAA coronas and hydrophobic PDEA cores. The morphology of the adsorbed layer was observed in situ using soft-contact atomic force microscopy (AFM): this technique suggests the formation of a thin adsorbed layer at pH 4 due to the adsorption of individual copolymer chains (unimers) rather than micelle aggregates. This is supported by the remarkably low dissipation values and the relatively low degrees of hydration for the adsorbed layers, as estimated using a combination of quartz crystal microbalance with dissipation monitoring (QCM-D) and optical reflectometry (OR). In alkaline solution, analysis of the adsorption data suggests a conformation for the adsorbed copolymers where one block projects normal to the solid/liquid interface; this layer consists of a hydrophobic PDEA anchor block adsorbed on the silica surface and an anionic PMAA buoy block extending into the solution phase. Tapping mode AFM studies were also carried out on the silica surfaces after removal from the copolymer solutions and subsequent drying. Interestingly, in these cases micelle-like surface aggregates were observed from both acidic and alkaline solutions. The lateral dimension of the aggregates seen is consistent with the corresponding hydrodynamic diameter of the copolymer micelles in bulk solution. The combination of the in situ and ex situ AFM data provides evidence that, for this copolymer, micelle aggregates are only seen in the ex situ dry state as a result of the substrate withdrawal and drying process. It remains unclear whether these aggregates are caused by micelle deposition at the surface during the substrate withdrawal from the solution or as a result of unimer rearrangements at the drying front as the liquid recedes from the surface.     

Irreversible adsorption of triple-helical soluble collagen monomers from solution to glass and other surfaces

The adsorption from various solutions of triple-helical soluble collagen monomers to solid surfaces was studied by labeling the collagen with ¹²⁵1. Adsorption to glass, siliconized glass, and Teflon, from aqueous solutions at various pH and ionic strength, was determined at collagen concentrations from 2 to 25 μg/ml. Adsorption was shown to be irreversible and little dependent on pH and ionic strength but increasing enormously as the surface is made more hydrophobic. Surface denaturation of the collagen by heat results in a substantial loss of material. The kinetics of adsorption suggest that the adsorption process may be selective and that not all collagen molecules which reach the surface are immediately adsorbed. Checking these results with earlier measurements of adsorbed layer thickness, a model for collagen adsorption is proposed.     

Adsorption of Bilirubin-Albumin Complex on the Surface of Highly Dispersed Silica

The interaction of bilirubin with albumin in aqueous buffer solutions is studied and the stability constants of the formed complex are determined. The adsorption of bilirubin and albumin from their individual and mixed solutions on the surface of highly dispersed silica is studied as a function of pH and albumin concentration. It is shown that the character of the adsorption of bilirubin from solutions containing albumin changes significantly compared to the adsorption from an individual solution.     

Thermal stability of organosilica and their forms modified by ions of copper(II)

Thermal stability of two-component organosilica sorbents – silico-polymethylsiloxanes with the ratio of hydrophilic and hydrophobic components of 50:50 and 70:30 mass%, and their forms, modified by copper(II), was investigated using thermal analysis (DTG, DTA, TG). The influence of the composition of modified organosilica sorbents and copper(II) content on the endothermic (removal of adsorbed water) and exothermic (decomposition by oxidation of methyl groups, possible hydration of surface) peak temperatures and on mass loss is shown. It was found that thermal stability of organosilica modified by copper(II) was lower than that of initial sorbents and depends on the content of modifying component and ratio of silanol and methylsilil groups in organosilica composition.     

Spreading of proteins and its effect on adsorption and desorption kinetics

The kinetics of adsorption of lysozyme and alpha-lactalbumin from aqueous solution on silica and hydrophobized silica has been studied. The initial rate of adsorption of lysozyme at the hydrophilic surface is comparable with the limiting flux. For lysozyme at the hydrophobic surface and alpha-lactalbumin on both surfaces, the rate of adsorption is lower than the limiting flux, but the adsorption proceeds cooperatively, as manifested by an increase in the adsorption rate after the first protein molecules are adsorbed. At the hydrophilic surface, adsorption saturation (reflected in a steady-state value of the adsorbed amount) of both proteins strongly depends on the rate of adsorption, but for the hydrophobic surface no such dependency is observed. It points to structural relaxation ("spreading") of the adsorbed protein molecules, which occurs at the hydrophobic surface faster than at the hydrophilic one. For lysozyme, desorption has been studied as well. It is found that the desorbable fraction decreases after longer residence time of the protein at the interface.     

Formation of a supramolecular gel between alpha-cyclodextrin and free and adsorbed PEO on the surface of colloidal silica: effect of temperature, solvent, and particle size

Aqueous solutions of alpha-cyclodextrin (alpha-CD) complex spontaneously with poly(ethylene oxide) (PEO), forming a supramolecular structure known as pseudopolyrotaxane. We have studied the formation of the complex obtained from the threading of alpha-CD onto PEO, both free in solution and adsorbed on colloidal silica. The kinetics of the reaction were studied by gravimetric methods and determined as a function of temperature and solvent composition for the PEO free in solution. PEO was then adsorbed on the surface of colloidal silica particles, and the monomers were displaced by systematically varying the degree of complexation, the concentration of particles, and the molecular weight of the polymer. The effect of the size of the silica particles on the yield of the reaction was also studied. With the adsorbed PEO, the complexation was found to be partial and to take place from the tails of the polymer. The formation of a gel network containing silica at high degrees of complexation was observed. Small-angle X-ray and neutron scattering experiments were performed to study the configuration of the polymeric chains and confirmed the partial desorption of the polymer from the surface of the silica upon complexation.     

Adsorption of plasmid DNA to a natural organic matter-coated silica surface: kinetics, conformation, and reversibility

A quartz crystal microbalance with dissipation (QCM-D) has been used to determine the adsorption rate of ampicillin-resistant linear and supercoiled plasmid DNA onto a silica surface coated with natural organic matter (NOM). The structure of the resulting adsorbed DNA layer was determined by analyzing the viscoelastic properties of the adsorbed DNA layers as they formed and were then exposed to solutions of different ionic composition. The QCM-D data were complemented by dynamic light scattering measurements of diffusion coefficients of the DNA molecules as a function of solution ionic composition. The obtained results suggest that electrostatic interactions control the adsorption and structural changes of the adsorbed plasmid DNA on the NOM-coated silica surface. The adsorption of DNA molecules to the NOM layer took place at moderately high monovalent (sodium) electrolyte concentrations. A sharp decrease in solution ionic strength did not result in the release of the adsorbed DNA, indicating that DNA adsorption on the NOM-coated silica surface is irreversible under the studied solution conditions. However, the decrease in electrolyte concentration influenced the structure of the adsorbed layer, causing the adsorbed DNA to adopt a less compact conformation. The linear and supercoiled DNA had similar adsorption rates, but the linear DNA formed a thicker and less compact adsorbed layer than the supercoiled DNA.     

Determination of Ag(I), Hg(II) and Pb(II) by using silica gel loaded with dithizone and zinc dithizonate

The modified sorbents with dithizone and zinc dithizonate adsorbed on the silica surface were obtained. The adsorption of heavy metal ions from aqueous solutions onto loaded silicas was studied. Color scales for Ag(I), Hg(II) and Pb(II) visual test detection were worked out. The modified silica gels were established to be applicable to semi-quantitative determination of these metal ions in buttermilk, natural, mineral and waste water.     

疏水多孔硅制备及其对水中有机污染物的吸附

以硅酸钠为硅源,盐酸为催化剂,三甲基氯硅烷(TMCS)为表面改性剂,经溶胶-凝胶和表面改性过程制备出一种疏水性多孔硅材料.采用傅里叶变换红外(FTIR)光谱仪、接触角分析仪、氮气物理吸附仪和扫描电子显微镜(SEM)对其结构和性质进行表征.结果表明:所制备的多孔硅具有分等级孔道结构(中孔-大孔),比表面积为566m2·g-1,孔体积高达2.28cm3·g-1,多孔硅与水的接触角为156°,显示出超疏水特征.对甲苯、汽油、柴油和润滑油的吸附量均可高达自身质量的14倍,丰富的孔道使其在几分钟内即可达到饱和吸附.这种多孔硅在汽油/水混合体系中对汽油具有较高的选择性,同时具有良好的再生能力.经正己烷萃取再生后,多孔硅仍能基本保持初始吸附容量.此方法制备的多孔硅材料在吸附分离污水中的有机物和溢油处理方面具有很好的应用前景.     

Molecular origin of anticooperativity in hydrophobic association

The structuring of water molecules in the vicinity of nonpolar solutes is responsible for hydrophobic hydration and association thermodynamics in aqueous solutions. Here, we studied the potential of mean force (PMF) for the formation of a dimer and trimers of methane molecules in three specific configurations in explicit water to explain multibody effects in hydrophobic association on a molecular level. We analyzed the packing and orientation of water molecules in the vicinity of the solute to explain the effect of ordering of the water around nonpolar solutes on many-body interactions. Consistent with previous theoretical studies, we observed cooperativity, manifested as a reduction of the height of the desolvation barrier for the trimer in an isosceles triangle geometry, but for linear trimers, we observed only anticooperativity. A simple mechanistic picture of hydrophobic association is drawn. The free energy of hydrophobic association depends primarily on the difference in the number of water molecules in the first solvation shell of a cluster and that in the monomers of a cluster; this can be approximated by the molecular surface area. However, there are unfavorable electrostatic interactions between the water molecules from different parts of the solvation shell of a trimer because of their increased orientation induced by the nonpolar solute. These electrostatic interactions make an anticooperative contribution to the PMF, which is clearly manifested for the linear trimer where the multibody contribution due to changes in the molecular surface area is equal to zero. The information theory model of hydrophobic interactions of Hummer et al. also explains the anticooperativity of hydrophobic association of the linear trimers; however, it predicts anticooperativity with a qualitatively identical distance dependence for nonlinear trimers, which disagrees with the results of simulations.     

Effect of bile salts on adsorption of cholesterol on silica adsorbent

The adsorption of cholesterol and several bile salts on the surface of modified hydrophobic silica adsorbent from individual and mixed solutions is studied at constant solution pH as a function of the concentration of bile salts. It is shown that, despite the solubilization of cholesterol by bile salts, cholesterol is adsorbed on the adsorbent as an individual compound, i.e. not as a complex with bile salts. The stronger the cholesterol is bound with bile salts, the smaller the value of adsorption. For the quantitative interpretation of adsorption data, a nonelectrostatic model of complexation on the surface is used.     

Effects of the nature and composition of the solvent on the thermodynamic characteristics of the individual forms of hydrogen adsorbed on the surface of porous nickel

The effects of the nature and composition of solvents on the thermodynamic characteristics of the adsorption states of hydrogen on nickel were studied. The adsorption can be described in terms of the thermodynamic model of a surface with a discrete nonhomogeneity for three individual forms of adsorbed hydrogen. The thermodynamic characteristics of the individual forms of hydrogen adsorbed on porous nickel from aqueous solutions of sodium hydroxide, dimethylformamide, methanol, and ethanol were determined.     

Effect of controlled deactivation on the thermochemical characteristics of hydrogen adsorption on skeletal nickel from sodium hydroxide-water solutions

Differential heats of adsorption in a wide range of surface coverage and maximum amounts of adsorbed hydrogen are determined by adsorption calorimetry on partially deactivated skeletal nickel from aqueous solutions of sodium hydroxide. The effect of the composition of solutions on the values of limiting adsorption and adsorption equilibria of individual forms of hydrogen is shown.     

Influence of the aggregation state in solution on the supramolecular organization of adsorbed type I collagen layers

In the last years, adsorbed collagen was shown to form layers with a supramolecular organization depending on the substrate surface properties and on the preparation procedure. If the concentration of collagen and the duration of adsorption are sufficient, fibrillar collagen structures are formed, corresponding to assemblies of a few molecules. This occurs more readily on hydrophobic compared to hydrophilic surfaces. This study aims at understanding the origin of such fibrillar structures and in particular at determining whether they result from the deposition of fibrils formed in solution or from the building of assemblies at the interface. Therefore, type I collagen solutions with an increasing degree of aggregation were prepared, using the “neutral-start” approach, by ageing pH 5.8 solutions at 37 °C for 15 min, 2 or 7 days. The obtained solutions were used to investigate the influence of collagen aggregation in solution on the supramolecular organization of adsorbed collagen layers, which was characterized by X-ray photoelectron spectroscopy and atomic force microscopy. Polystyrene and plasma-oxidized polystyrene were chosen as substrates for the adsorption. The size and the density of collagen fibrils at the interface decreased upon increasing the degree of aggregation of collagen in solution. This is explained by a competitive adsorption process between monomers and aggregates of the solution, turning at the advantage of the monomers. More aggregated solutions, which are thus depleted in free monomers, behave like less concentrated solutions, i.e. lead to a lower adsorbed amount and less fibril formation at the interface. This study shows that the supramolecular fibrils observed in adsorbed collagen layers, especially on hydrophobic substrates, are not formed in the solution, prior to adsorption, but are built at the interface, through the assembly of free segments of adsorbed molecules.     

Preconcentration of benzalkonium chloride using sodium dodecyl sulfate attached to a strong anion exchange resin

A surfactant-mediated solid phase extraction procedure is applied for the preconcentration of benzalkonium (BA) chloride from a river water sample. Dodecyl sulfate is attached to a strong anion exchange resin and aqueous samples are passed through a column containing this surfactant-resin material. Benzalkonium chloride, a cationic compound very useful in cosmetics and an important fungicide, is adsorbed from the aqueous solution onto the sorbent via hydrophobic and electrostatic interactions. When using traditional silica-based nonpolar sorbents, strong electrostatic interactions between the cationic analyte and the silica surface make elution difficult. Using the presented sorbent, electrostatic attractions occur between the benzalkonium cations and removable dodecyl sulfate anions. Removing this ion pair from the sorbent results in efficient elutions. The results of this solid-phase extraction (SPE) method are presented in terms of various rinse solutions parameters, breakthrough studies and a real river water sample.     

Microcontact printing of Alzheimer’s β-amyloid monomers and fibrils

Adsorption of the 40-residue Alzheimer’s β-amyloid peptide (Aβ40) on a hydrophobic surface leads to formation of potentially disease-relevant aggregates. Existing techniques are limited in characterizing the adsorbed Aβ40 and producing potentially useful Aβ40 microstructures such as microarrays and microparticles. In this paper, a novel approach based on microcontact printing (μCP) to studying and utilizing adsorption of Aβ40 monomers and fibril fragments on hydrophobic surface of polydimethylsiloxane (PDMS) stamps has been developed. By transferring the adsorbed layer from the stamp to a glass substrate, this approach allows easy measurement of thickness of the adsorbed layer. It also enables characterization of the face of the adsorbed layer in contact with the stamp surface. This face exhibits significant higher thioflavin T fluorescence than the face exposed to water, suggesting β-sheet formation induced by the PDMS surface. The intrinsic stability of the adsorbed layer is evaluated by printing the layer on a water-soluble substrate and exposing it to water vapor or water. Stable particulate microstructures in water are obtained by chemically crosslinking the adsorbed peptides. Moreover, co-micropatterning of the different states of Aβ40 (monomers and fibril fragments) is demonstrated. This μCP-based approach is simple, versatile, and holds potential for various applications.