Binary adsorption of globular proteins on ion-exchange media

Adsorption equilibrium of binary pairs of lysozyme (LYS), cytochrome c (CYC) and ribonuclease A (RNase) has been measured on different cation-exchange media at various solution conditions. Adsorption patterns largely follow the intrinsic protein–surface interactions, but can differ significantly for different pairs or even for one pair at different solution conditions. LYS/CYC adsorption shows similar behavior on all the adsorbents examined, with competitive adsorption dominated by LYS and the presence of LYS reducing the adsorption of CYC significantly. Simultaneous and sequential measurements for LYS/CYC show that the order of adsorption does not have a significant effect on the adsorption equilibrium. For LYS/RNase, LYS is consistently more strongly adsorbed. For CYC/RNase, both proteins can display significant adsorption, depending on the pH and salt concentration. A model based on colloidal energetics is developed to calculate the binary adsorption isotherms using parameter values obtained from single-component isotherms. The calculated adsorption is in good agreement with experimental results, with significantly better representation than for other commonly used binary isotherms.     

Adsorption of deamidated antibody variants on macroporous and dextran-grafted cation exchangers: II. Adsorption kinetics

Single and multicomponent batch adsorption kinetics were obtained for deamidated mAb variants on two commercial cation exchangers, one with an open macroporous structure--UNOsphere S--and the other with charged dextran grafts--Capto S. The adsorption kinetics for the macroporous matrix was found to be controlled largely by pore diffusion. The effective diffusivity estimated from single component data was a fraction of the mAb free solution diffusivity, and its value could be used to accurately predict the adsorption kinetics for two- and three-component systems. In this case, when two or more variants were adsorbed simultaneously, both experimental and predicted results showed a temporary overshoot of the amount adsorbed above the equilibrium value for the more deamidated variant followed by a gradual approach to equilibrium. Adsorption rates on the dextran grafted material were much faster than those observed for the macroporous matrix for both single component and simultaneous adsorption cases. In this case, no significant overshoot was observed for the more deamidated forms. The Capto S adsorption kinetics could be described well by a diffusion model with an adsorbed phase driving force for single component adsorption and for the simultaneous adsorption of multiple variants. However, this model failed to predict the adsorption kinetics when more deamidated forms pre-adsorbed on the resin were displaced by less deamidated ones. In this case, the kinetics of the displacement process was much slower indicating that the pre-adsorbed components severely hindered transport of the more strongly bound variants. Overall, the results indicate that despite the lower capacity, the macroporous resin may be more efficient in process applications where displacement of one variant by another takes place as a result of the faster and more predictable kinetics.     

Examining the accuracy of ideal adsorbed solution theory without curve-fitting using transition matrix Monte Carlo simulations

Ideal adsorbed solution theory (IAST) is a well-known approach to predicting multicomponent adsorption isotherms in microporous materials from experimental or simulation data for single-component adsorption. A limitation in practical applications of IAST is that useful calculations often require extrapolation of fitted single-component isotherms beyond the range for which data are available. We introduce a molecular simulation approach in which the intrinsic accuracy of IAST can be examined in a context that avoids any need to perform curve fitting with single-component data. Our approach is based on using transition matrix Monte Carlo to define single-component adsorption isotherms for arbitrary bulk-phase pressures from a single simulation. We apply our approach to several light gas mixtures in silica zeolites and a carbon nanotube to examine the intrinsic accuracy of IAST for these model systems.     

Two-component protein adsorption to the cation exchanger S Sepharose FF

A model system, consisting of bovine serum albumin, lysozyme and the cation exchanger S Sepharose FF, was used to investigate multicomponent protein adsorption to ion exchangers. Two models, one based on a complete absence of competition between adsorbing molecules and the other a competitive model, based on the assumption that all adsorption sites are available to both proteins, have been compared to experimental results. Evidence for competitive adsorption was seen in experiments in which breakthrough curves and the profiles of adsorbed proteins in packed beds were determined. However, although the results for packed-bed experiments were more closely predicted by the fully competitive model, some discrepancies were found suggesting that when considering multicomponent protein adsorption to ion exchangers it may also be necessary to take account of factors such as the molecular size of the adsorbing proteins and any potential inter-protein interactions, factors which may hinder the development of a general model of multicomponent protein adsorption to ion exchangers.     

Quantitative analysis of protein adsorption on a planar surface by Fourier transform infrared spectroscopy: lysozyme adsorbed on hydrophobic silicon-containing polymer

The adsorption of hen egg white lysozyme onto a solid polytris(trimethylsiloxy)silylstyrene (pTSS) surface from a D(2)O solution at pD 7 containing 100 mM NaCl and 10 mM sodium deuterated phosphate was monitored at 25 degrees C by Fourier transform infrared spectroscopy using the attenuated total reflection (ATR) method. The infrared spectrum attributed to only the adsorbed lysozyme was derived from the observed spectrum, and the amount of adsorbed lysozyme was determined as a function of time and lysozyme concentration. The kinetics of adsorption could be decomposed into two components, one of which was a process with a time constant of larger than 4 h(-1) and the other was a process with one of about 0.1 h(-1). These spectra showed that the lysozyme adsorbed in the faster process had a higher beta-structure content than the dissolved lysozyme. It was also found that the slower adsorption induced some conformational change in the lysozyme adsorbed in the faster process and/or that adsorbed in the slower process. After adsorption for 24 h, the pTSS surface was rinsed out with lysozyme-free solution. The resultant spectra of the surface indicated that the lysozyme adsorbed in the faster process was bound irreversibly on the surface and was changed to a conformer with a higher beta-structure content during the slower process. The experimental procedures and the theoretical applications for such a quantitative analysis in the ATR spectroscopic method are presented in detail.     

Exploring the interfacial structure of protein adsorbates and the kinetics of protein adsorption: an in situ high-energy X-ray reflectivity study

The high energy X-ray reflectivity technique has been applied to study the interfacial structure of protein adsorbates and protein adsorption kinetics in situ. For this purpose, the adsorption of lysozyme at the hydrophilic silica-water interface has been chosen as a model system. The structure of adsorbed lysozyme layers was probed for various aqueous solution conditions. The effect of solution pH and lysozyme concentration on the interfacial structure was measured. Monolayer formation was observed for all cases except for the highest concentration. The adsorbed protein layers consist of adsorbed lysozyme molecules with side-on or end-on orientation. By means of time-dependent X-ray reflectivity scans, the time-evolution of adsorbed proteins was monitored as well. The results of this study demonstrate the capabilities of in situ X-ray reflectivity experiments on protein adsorbates. The great advantages of this method are the broad wave vector range available and the high time resolution.     

Molecular Modeling of Adsorption in Activated Carbon: Comparison of Monte Carlo Simulations with Experiment

We present Mont Carlo computer simulation results for a molecular model of fluids adsorbed in porous carbon materials. The model carbon used is based on the platelet model for carbon of Segarra and Glandt (1994). The model we use has a single basal plane per platelet and the structure is isotropic, disordered, with weak short-range correlations between the platelets. We have performed grand canonical Monte Carlo simulations of the adsorption isotherms for methane, ethane, and their mixtures in this model carbon. We find generally good agreement with experimental and the mixture results are quite accurately described by the ideal adsorbed solution theory. An exception to this is the behavior for the mixture at the highest pressures. In this case the experimental data show significant deviations from ideal adsorbed solution theory and the simulation results.     

细胞色素c在经预处理的金电极上电化学行为的研究

本文观察到吸附态和本体态的细胞色素ｃ以经预处理的金电极上的准可逆反应。采用现场ＦＴＩＲ光谱法，紫外可见反射光谱法，循环伏安法，交流阻抗法和电位阶跃法研究了细胞色素ｃ吸附行为和反应动力学。     

Adsorption and structure of hydrocarbons in MCM-41: a computational study

The adsorption of linear, branched, and cyclic hydrocarbons in MCM-41 is studied using Configurational Bias Monte Carlo simulations. A new computational model for MCM-41 is proposed which, although simple, is able to predict adsorption isotherms which are in agreement with the scarce experimental data. The structure of the adsorbed phase is analyzed and found to be similar to that of studies using small, hard spheres trapped in pores. The adsorption of mixtures is investigated, and the adsorption hierarchy is discussed. The structure of the adsorbed mixture is revealed and shows that all components of the mixture exhibit structure, even if they are only adsorbed in small quantities. Finally, the model is modified to include surface roughness and the effect on the adsorption isotherms and structure of the adsorbed phase is discussed.     

Proteins at liquid interfaces: II. Adsorption isotherms

Adsorption isotherms for the three proteins β-casein, bovine serum albumin, and lysozyme at the air-water and oil-water interfaces have been determined independently using ellipsometry and surface radioactivity methods; the surface pressure and surface potential were also monitored. Saturated monolayer coverage occurs via irreversible adsorption of 2–3 mg M^?2 of protein; the resultant films generate surface pressures of about 20 mN m^?1 and are 50–60 Å thick. Molecules adsorbed in the first layer dominate the film pressures so that further adsorption causes no change in the pressure although the film thickness can increase to more than 100 Å. The molecules which give rise to this increase in film thickness are reversibly adsorbed with respect to aqueous substrate exchange. The experimental isotherm data and the Langmuir adsorption isotherm are in close agreement at low protein concentrations. However, comparison with the Gibbs adsorption equation is not valid, although reasonable agreement can be achieved if some account is taken of the fact that the protein molecules in the first layer are irreversibly adsorbed.     

Testing the accuracy of correlations for multicomponent mass transport of adsorbed gases in metal-organic frameworks: diffusion of H2/CH4 mixtures in CuBTC

Mass transport of chemical mixtures in nanoporous materials is important in applications such as membrane separations, but measuring diffusion of mixtures experimentally is challenging. Methods that can predict multicomponent diffusion coefficients from single-component data can be extremely useful if these methods are known to be accurate. We present the first test of a method of this kind for molecules adsorbed in a metal-organic framework (MOF). Specifically, we examine the method proposed by Skoulidas, Sholl, and Krishna (SSK) ( Langmuir, 2003, 19, 7977) by comparing predictions made with this method to molecular simulations of mixture transport of H 2/CH 4 mixtures in CuBTC. These calculations provide the first direct information on mixture transport of any species in a MOF. The predictions of the SSK approach are in good agreement with our direct simulations of binary diffusion, suggesting that this approach may be a powerful one for examining multicomponent diffusion in MOFs. We also use our molecular simulation data to test the ideal adsorbed solution theory method for predicting binary adsorption isotherms and a method for predicting mixture self-diffusion coefficients.     

Band splitting in overloaded isocratic elution chromatography II. New competitive adsorption isotherms

The equations of two new binary competitive isotherms models are derived. The first of these models assumes that the isotherms of the two pure, single compounds have distinct monolayer capacities. Its derivation is based on kinetic arguments. The ideal adsorbed solution (IAS) framework was applied to derive the second model that is a thermodynamically consistent competitive isotherm. This second model predicts the competitive adsorption isotherm behavior of a mixture of two compounds that have single-component adsorption behavior following a BET and/or a Langmuir isotherms. Both models apply well to the binary adsorption of ethylbenzoate and 4-tert.-butylphenol on a Kromasil-C18 column (with methanol-water, 62:38, v/v, as the mobile phase). The best single-solute adsorption isotherms of these two compounds are the liquid-solid extended multilayer BET and the Langmuir isotherms, respectively. The kinetic and thermodynamic new competitive models were compared, regarding the accuracy of their prediction of the elution band profiles of mixtures of these two compounds. A better agreement between experimental and calculated profiles was observed with the kinetic model. The IAS model failed because the behavior of the ethylbenzoate/4-tert.-butylphenol adsorbed phase mixture is probably non-ideal. The most striking result is the qualitative prediction by these models of the peak splitting of 4-tert.-butylphenol during its elution in presence of ethylbenzoate.     

Application of the vacancy solution theory to describe the enthalpic effects accompanying mixed-gas adsorption

The possibility of utilizing vacancy solution theory (VST) to study the enthalpic effects accompanying mixed-gas adsorption equilibria is presented. Besides heterogeneity, the interaction effects by using the regular adsorbed solution, Flory-Huggins, and Wilson models of nonideality in the adsorbed phase are taken into account. To predict adsorption phase diagrams and calorimetric effects in the mixed-gas adsorption system, only a knowledge of the single-gas adsorption isotherms and accompanying calorimetric effects is required. The possibility of simplification of the obtained theoretical expressions is shown. The obtained agreement between theory and experiment is very satisfactory.     

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.     

New thermodynamically consistent competitive adsorption isotherm in RPLC

A new equation of competitive isotherms was derived in the framework of the ideal adsorbed solution (IAS) that predicts multisolute adsorption isotherms from single-solute isotherms. The IAS theory makes this new isotherm thermodynamically consistent, whatever the saturation capacities of these single-component isotherms. On a Kromasil-C(18) column, with methanol-water (80/20 v/v) as the mobile phase, the best single-solute adsorption isotherm of both toluene and ethylbenzene is the liquid-solid extended multilayer BET isotherm. Despite a significant difference between the monolayer capacities of toluene (370 g/l) and ethylbenzene (170 g/l), the experimental adsorption data fit very well to single-component isotherms exhibiting the same capacities (200 g/l). The new competitive model was used for the modeling of the elution band profiles of mixtures of the two compounds. Excellent agreement between experimental and calculated profiles was observed, suggesting that the behavior of the toluene-ethylbenzene adsorbed phase on the stationary phase is close to ideal. For example, the concentrations measured for the intermediate plateau obtained in frontal analysis differ by less than 2% from those predicted by the IAS model.     

On the Use of the Hypothesis of Statistically Homogeneous Suspensions in the Calculation of Their Conductivity

Electrostatic effects on protein adsorption were investigated using differential scanning calorimetry (DSC) and adsorption isotherms. The thermal denaturation of lysozyme, ribonuclease A (RNase), and alpha-lactalbumin in solution and adsorbed onto silica nanoparticles was examined at three concentrations of cations: 10 and 100 mM of sodium and 100 mM of sodium to which 10 mM of calcium was added. The parameters investigated were the denaturation enthalpy (DeltaH), the temperature at which the denaturation transition was half-completed (T(m)), and the temperature range of the denaturation transition. For lysozyme and RNase, adsorption isotherms depend strongly on the ionic strength. At low ionic strength both proteins have a high affinity for the silica particles and adsorption is accompanied by a 15-25% reduction in DeltaH and a 3-6 degrees C decrease in T(m), indicating that the adsorbed state of the proteins is destabilized. Also, an increase in the width of the denaturation transition is observed, signifying a larger conformational heterogeneity of the surface bound proteins. At higher ionic strengths, both with and without the addition of calcium, no significant adsorption-induced alteration in DeltaH was observed for all three proteins. The addition of calcium, however, decreases the width of the denaturation transition for lysozyme and RNase in the adsorbed state. Copyright 2001 Academic Press.     

混合气体中SAPO-34上单个物种的表面浓度简

运用Langmuir等温线方程和理想吸附溶液理论（IAST）两种方法计算了SAPO-34在混合气体中的单个物种表面浓度,并对比了计算值与实验值的吻合程度. 考察了两个二元混合体系,分别为80 ℃的甲醇和二甲醚以及25 ℃的二甲醚和乙烯混合气,发现IAST计算值在实验压力范围内均与实验结果吻合;但是Langmuir理论计算值仅在酸性位覆盖率低于1/3时与实验值吻合较好,随着压力增加严重偏离实验值,而且Langmuir理论不能描述随压力增加低饱和吸附量物种覆盖率降低的现象. 因此,针对包含不同饱和吸附量组分的混合气,Langmuir理论仅适用于描述表面浓度低时的反应动力学,当表面浓度高时应该采用IAST方法.     

Application of mass spectrometry for study of the adsorption of multicomponent surfactant mixtures at the solid/solution interface

The selective adsorption of the components of a polydisperse gemini surfactant blend (alkylbenzenesulfonate-Jeffamine salt, ABSJ) in aqueous solution onto Berea sandstone, a reference material in enhanced oil recovery (EOR), was investigated. The individual adsorption isotherms of the four, benzene-ring containing ABSJ components with different alkyl chain lengths (ranging from decyl to tridecyl of the alkyl chain length) were simultaneously determined by using a four-channel electrospay ionization mass spectrometer (ESI-MS) for concentration analysis. This analytical device provided selective information (based on the differences in the mass to charge ratio) on the adsorption of each component in the mixed surfactant system. The overall isotherm obtained from the superposition of the individual isotherms determined by ESI-MS agreed well with the isotherm determined by UV spectrometry; the UV equipment is benzene-ring sensitive, irrespective of the alkyl chain length. The S-shaped isotherms reached a plateau at the critical micelle concentration. Longer-chain surfactants adsorbed preferentially over the short chain homologs, independently of solution concentration. This analytical device provided the net adsorption isotherm. Most analytical methods are not component selective, and thus they are not able to measure the individual isotherms in multicomponent solutions. Here, we report on a novel method which describes the selective determination of the individual adsorption isotherms of surfactants in a multicomponent mixture. The theoretical background of the method is described in detail.     

Concentration effects on adsorption of bacteriophage T4 lysozyme stability variants to silica

The adsorption kinetics and dodeceyltrimethylammonium bromide-mediated elution of the wild type and two structural stability mutants of bacteriophage T4 lysozyme were recorded in situ, at silica surfaces. Experiments were performed at different solution concentrations, ranging from 0.01 to 1.0 mg/ml. Plateau values of adsorbed mass generally increased with increasing solution concentration, with the adsorbed layer being only partially eluted by buffer. Treatment with surfactant removed more of the adsorbed protein in each case, with the remaining adsorbed mass varying little with concentration. Comparison of the data to an adsorption mechanism allowing for three adsorbed states, distinguished by binding strength, showed that the fraction of adsorbed molecules present in the most tightly bound state (state 3) decreased as adsorption occurred from solutions of increasing concentration. However, the absolute amounts of state 3 molecules present in each case were less dependent on solution concentration. Adsorption of T4 lysozyme into state 3 is suggested to occur early in the adsorption process and continue until some critical surface concentration is reached. Beyond this critical value of adsorbed mass, adsorption is suggested to progress with adoption of more loosely bound states.     

Measurement of Gas Mixture Adsorption Equilibria of Natural Gas Compounds on Microporous Sorbents

Physisorption equilibria of multicomponent gases on microporous solids like zeolites or activated carbons are considered. In view of lack of reliable and simple methods to calculate mixture adsorption isotherms from pure component data, experiments are still indispensable. An overview of classical and new methods to measure multicomponent gas adsorption equilibria is given. Some of the basic concepts like the Gibbs excess mass and the absolute mass adsorbed underlying these methods are discussed. Experimental data and a class of new adsorption isotherms for inhomogeneous microporous adsorbents of fractal dimension will be given in another subsequent paper (ADSO 635-98) by the same group of authors.