Adsorption kinetics of beta-lactoglobulin on a polyclonal immunochromatographic support

Separation Technologies for Whey Protein Fractionation

Whey is a by-product of cheese, casein, and yogurt manufacture. It contains a mixture of proteins that need to be isolated and purified to fully exploit their nutritional and functional characteristics. Protein-enriched fractions and highly purified proteins derived from whey have led to the production of valuable ingredients for many important food and pharmaceutical applications. This article provides a review on the separation principles behind both the commercial and emerging techniques used for whey protein fractionation, as well as the efficacy and limitations of these techniques in isolating and purifying individual whey proteins. The fractionation of whey proteins has mainly been achieved at commercial scale using membrane filtration, resin-based chromatography, and the integration of multiple technologies (e.g., precipitation, membrane filtration, and chromatography). Electromembrane separation and membrane chromatography are two main emerging techniques that have been developed substantially in recent years. Other new techniques such as aqueous two-phase separation and magnetic fishing are also discussed, but only a limited number of studies have reported their application in whey protein fractionation. This review offers useful insights into research directions and technology screening for academic researchers and dairy processors for the production of whey protein fractions with desired nutritional and functional properties.

Understanding protein adsorption phenomena at solid surfaces

Protein adsorption at solid surfaces plays a key role in many natural processes and has therefore promoted a widespread interest in many research areas. Despite considerable progress in this field there are still widely differing and even contradictive opinions on how to explain the frequently observed phenomena such as structural rearrangements, cooperative adsorption, overshooting adsorption kinetics, or protein aggregation. In this review recent achievements and new perspectives on protein adsorption processes are comprehensively discussed. The main focus is put on commonly postulated mechanistic aspects and their translation into mathematical concepts and model descriptions. Relevant experimental and computational strategies to practically approach the field of protein adsorption mechanisms and their impact on current successes are outlined.

Development of a fast and simple immunochromatographic method to purify alpha 1-acid glycoprotein from serum for analysis of its isoforms by capillary electrophoresis

Alpha 1-acid glycoprotein (AGP) is a very heterogeneous glycoprotein presenting several isoforms due to variations in its polypeptidic and glycosidic moieties. Differences in AGP isoforms between healthy and diseased individuals have been related to different pathological situations such as cancer or cardiovascular diseases, among others. Capillary electrophoresis study of the role of AGP isoforms as biomarkers requires prior purification of AGP from biological samples. Current AGP purification methods are time- and labour-consuming, and generally they have not been proven to be compatible with capillary electrophoresis analysis. In this work, different methods for AGP purification from human serum are developed and compared. The applicability of acidic precipitation and immunoaffinity chromatographic methods for AGP purification are studied. Two different immunoaffinity approaches are employed; in the first one, interferents present in the AGP sample are captured and removed, and in the second one, AGP is retained in a house-made anti-AGP column, being in this way isolated from the rest of interferents of the sample. Best results in AGP purification from human serum to be analyzed by capillary zone electrophoresis (CZE) were obtained when acidic purification was combined with immunoaffinity chromatography (IAC) employing the house-made anti-AGP column. The method was shown not to alter the proportion of AGP peaks due to isoforms existing in AGP samples. The applicability of this fast and easy purification method developed for analyzing by CZE isoforms of AGP from natural serum samples by CZE is demonstrated.

Kinetic studies of biological interactions by affinity chromatography

The rates at which biological interactions occur can provide important information on the mechanism and behavior of such processes in living systems. This paper will discuss how affinity chromatography can be used as a tool to examine the kinetics of biological interactions. This approach, referred to here as biointeraction chromatography, uses a column with an immobilized binding agent to examine the association or dissociation of this agent with other compounds. The use of HPLC-based affinity columns in kinetic studies has received particular attention in recent years. Advantages of using HPLC with affinity chromatography for this purpose include the ability to reuse the same ligand within a column for a large number of experiments, and the good precision and accuracy of this approach. A number of techniques are available for kinetic studies through the use of affinity columns and biointeraction chromatography. These approaches include plate height measurements, peak profiling, peak fitting, split-peak measurements, and peak decay analysis. The general principles for each of these methods are discussed in this paper and some recent applications of these techniques are presented. The advantages and potential limitations of each approach are also considered.

Retention properties of hydrophobically end-capped poly(ethylene glycol)s on a beta-cyclodextrin support

High-performance liquid chromatography (HPLC) was used to examine the retention behavior of monomethoxypoly(ethylene glycol)s bearing one hydrophobic naphthyl end group (Nap-MPEG) on beta-cyclodextrin polymer (poly-beta-CD) immobilized on a silica support, under isocratic elution conditions and using water as mobile phase. Studies of retentions and theoretical plate heights H were conducted at infinite dilution by comparing the behavior of Nap-MPEGs having different molecular weight (750, 1000 and 5000 g/mol). The larger is its molecular size, the lower is the retention of the polymer. The linear increase of H with mobile phase velocity reveals slow mass-transfer kinetics arising from the restricted diffusion into the pores of the support. The complexation constants between the Nap-MPEGs and beta-CD in solution (around 500M(-1)) were determined from the decrease of retention observed by adding increasing concentrations of hydroxypropyl beta-CD into the eluent. The peak profiles in mass-overload conditions were studied by fitting a model based upon bi-Langmuir kinetics which assumes a non-uniform support having two types of binding sites and apparent adsorption rate constants are used to describe mass-transfer kinetics. A three-parameter adsorption equilibrium isotherm was sufficient to account for the modifications of peak shapes observed when increasing amounts of polymer were injected. This result indicates an interaction with a heterogeneous poly-beta-CD support mainly composed of low affinity groups, non-saturable in the range of polymer concentration studied. An upper limit was estimated for the equilibrium constant (<1000 M(-1)) characterizing the affinity of Nap-MPEG for the non-saturable sites of the poly-beta-CD support. Large affinity constants (8-9 x 10(4)M(-1)) were found for the interaction of Nap-MPEGs with a small percentage of active sites.

Frontal analysis for characterizing the adsorption–desorption behavior of β-lactoglobulin on immunoadsorbents

High-performance frontal affinity chromatography was employed to study the adsorption-desorption kinetics characterizing the retention of beta-lactoglobulin (beta-LG) onto polyclonal anti-beta-lactoglobulin (anti-beta-LG) chromatographic supports. The adsorption and desorption processes were studied by analyzing two different elution fronts separated by a relatively long rinsing step. The method consists in performing two successive frontal injections of the protein. In between, the column was rinsed with a given volume of mobile phase (buffer alone). During this rinsing stage, a partial desorption may occur and a novel amount of protein could be adsorbed in the second frontal injection step. The whole process (first adsorption, possible desorption, and second adsorption) was simulated by a numerical procedure, in which the column was divided into a large number of slices. A model based on bi-Langmuir type kinetics was used to describe the adsorption of the protein on the support. The model assumes a non-uniform adsorbent with two types of binding sites. At equilibrium the adsorption isotherm is of the bi-Langmuir type. A global adsorption effect was considered which includes the effective binding process and the mass transfer resistances due to the transport to the binding site. Therefore, the column capacity and the kinetic parameters of the model (apparent adsorption and desorption rate constants) were determined from the best fit of the first and second adsorption fronts to the experimental ones. The other parameters of the model are the saturation capacities for the adsorption on each type of sites. The equilibrium affinity constants were estimated in a single experiment from the ratio of the apparent adsorption and desorption rate constants. The high values found (around 10(8) M(-1)) reveal a strong interaction of beta-LG with the immunoadsorbent. Kinetic measurements were carried out at different flow rates. Both the apparent adsorption and desorption kinetics were faster at larger flow rates, indicating an important contribution of the mass transfer resistance in the stagnant fluid at the particle boundary. However, as expected, close values were found for the resulting equilibrium constants calculated from the ratio of the apparent adsorption and desorption rate constant determined at various flow rates.

Electrochemical Quartz Crystal Nanobalance to Detect Solvent Displacement by pH-Induced Conformational Changes of Proteins at Pt

The electrochemical quartz crystal nanobalance (EQCN) techniques of simultaneous measurements of frequency and cyclic voltammetry (CV) were used to investigate protein adsorption behavior resulting from pH-induced conformational changes at the Pt electrode at 298 K. The adsorption behavior of holo- and apo-alpha-lactalbumin was studied in electrolyte solutions of pH < 2, 7.4, and 11. The EQCN frequency measurements did not directly monitor the mass of the adsorbed protein at anodic potentials, but instead, at a potential characteristic of the double layer for platinum, gave a measure of the extent of solvent displacement by the adsorbed protein (i.e., a "footprint"), which correlated well with known pH-induced conformational changes of the protein. Simultaneous CV charge transfer measurements provided information on the number of layers of protein adsorbed to the surface. This ability of the EQCN to detect solvent displacement by protein adsorption is potentially useful for biosensors to detect and to monitor protein conformational changes in the bulk and during the adsorption process. The Langmuir adsorption isotherm provided the Gibbs energy of adsorption, DeltaG(ADS), and showed excellent agreement between the CV and EQCN frequency measurements.

Use of immunodotting to select the desorption agent for immunochromatography

In immunochromatography, a technique of increasing use, the sample containing the antigen (Ag) to be purified or determined is introduced into a chromatographic column containing an antibody (Ab) bound to the packing material. The antigen is retained based on antigen-antibody recognition. To reuse the immunocolumn for subsequent assays, the antigen has to be eluted without causing irreversible damage of antibodies. Selection of conditions for performing immunochromatography is usually made by trial and error. This way of working is time consuming and it may ruin the column. In this article, the feasibility of using immunodotting to select the conditions to be employed in one immunochromatographic assay is shown. An immunodotting method is developed to select the best desorption agent for an enzyme-linked immunoaffinity chromatography (ELIAC) assay to determine beta-lactoglobulin (beta-LG). The effect of several factors on the immunodotting performance is studied. The way of performing solvent exchange to treat the antibody with different solutions considered as potential desorption agents to check their effect is shown. Effectiveness of the solution chosen by immunodotting (4 M MgCl(2) in 20 mM Tris, pH 5.9) as desorption agent is demonstrated by immunochromatographic assays. The immunodotting and solvent exchange methods developed should be useful to choose solvents and conditions for any other kind of assay.