Performance evaluation of a focusing buffer developed for chromatofocusing on high-performance anion-exchange columns

Isoelectric point separation of proteins by capillary pH-gradient ion-exchange chromatography

In the present work, isoelectric point (pl) separation of proteins by pH-gradient ion-exchange chromatography (IEC) on packed capillary columns is demonstrated. The development of a miniaturized flow-through pH probe for reliable pH monitoring of the column effluent, which was an important technical challenge for adapting this technique to capillary dimensions, was solved by designing a low microliter per minute flow rate housing to a commercially available micro pH probe. Highly linear outlet pH-gradients within the pH range 8.5-4.0 were obtained when applying simple inexpensive buffers consisting solely of piperazine, N-methylpiperazine and imidazole on 10 cm x 0.32 mm i.d. fused silica capillaries packed with anion-exchange poly(styrene divinylbenzene)-based macroporous materials, i.e. 10 microm Mono P from Amersham Biosciences and 10 microm PL-SAX from PolymerLabs. Furthermore, when using a pH-gradient from 6.8 to 4.3, both columns were able to baseline separate the A and B genetic variants of beta-lactoglobulin, which differ with two amino acid residues only, but the PL-SAX column provided almost a two-fold decrease in peak widths compared to the Mono P column. The influence of varying the buffer concentration, injection volume and column temperature on the peak widths and resolution of the beta-lactoglobulins was investigated, e.g. a 100 microl sample of dilute beta-lactoglobulins was injected directly on the column with practically no increase in peak width as compared to what obtained with conventional injection volumes. Finally, a pH-gradient from 6.8 to 4.3 was used to separate proteins in skimmed bovine milk on the PL-SAX column. The milk was simply diluted 1:10 (v/v) with water and filtrated before injection.

Hydrophobic character of surface regions and total hydrophobicity of four variants of chromosomal class C beta-lactamase from Pseudomonas aeruginosa are identical. Chromatographic comparison of the hydrophobic character of the variants and the effect of focusing buffer composition on the separation of the variants by chromatofocusing with internal and external pH gradients

The hydrophobic character of class C beta-lactamase molecular variants from Pseudomonas aeruginosa was compared by hydrophobic interaction chromatography and reversed-phase liquid chromatography, respectively. Separation of the variants by hydrophobic interaction chromatography was not achieved by modifying salt and pH of mobile phases. Reversed-phase liquid chromatography of the variants resulted in almost identical retention times. The results showed that the hydrophobic character of surface regions as well as total hydrophobicity of the variants are identical. The resolving power of external, internal and gradient chromatofocusing of the variants on strong and weak anion exchangers using low-molecular-mass buffers was compared to that of commercial ampholytes and showed no difference in separation pattern of the variants. Comparisons of variant isoelectric point (pI) values determined by chromatofocusing and isoelectric focusing showed that pI values determined by gradient chromatofocusing were most similar to the pI values determined by isoelectric focusing.

High-performance chromatofocusing using linear and concave pH gradients formed with simple buffer mixtures. I. Effect of buffer composition on the gradient shape

Numerical calculations together with simplified analytical relations based on local equilibrium theory are used to determine the factors which govern the shape of the gradient formed during chromatofocusing when simple mixtures of buffering species are employed to produce linear or concave pH gradients. The numerical and analytical development is also used to determine the relation between the gradient shape and the buffering capacities of the adsorbed and liquid phases. Experiments which verify the theoretical methods are described where internally generated, retained pH gradients of various shapes are formed using high-performance chromatography columns. The resulting experimental and theoretical basis can be employed as means for the selection of the buffer composition for use in chromatofocusing.

Displacement chromatography of proteins using a self-sharpening pH front formed by adsorbed buffering species as the displacer

The preparative-scale separation of two proteins into adjoined, pure bands was accomplished using a novel, hybrid chromatography method which employs chromatofocusing using a self-sharpening pH front and displacement development. The method eliminates the use of a traditional displacer for accomplishing displacement chromatography, and was used to separate the A and B forms of beta-lactoglobulin using a strong-base anion-exchange column packing and a buffer system composed of acetic acid and either 3-(N-morpholino)propane-sulfonic acid (MOPS) or 2-(N-morpholino)ethanesulfonic acid (MES). Sample loads up to 150 mg of protein were applied to a 75 x 7.5 mm column to produce a displacement train composed of highly pure protein bands with greater than 90% recovery of protein. A discussion is given of the chromatographic behavior of proteins under concentration overloaded conditions for the case where a self-sharpening pH front formed using adsorbed buffering species is used to desorb proteins from an anion-exchange column packing.

Quasi-linear pH gradients for chromatofocusing using simple buffer mixtures: local equilibrium theory and experimental verification

Chromatofocusing utilizes internally generate, retained pH gradients to focus proteins into narrow chromatographic bands. One of the characteristics of current chromatofocusing methods is that they use expensive polyampholyte buffers containing large numbers of ill-defined components in order to generate linear or quasi-linear pH gradients. In addition to being costly to use, polyampholyte buffers also tend to associate with proteins and often yield irreproducible gradient shapes. In order to avoid the various difficulties associated with the use of polyampholyte buffers, this study investigates the use of mixtures of simple buffering species to generate quasi-linear pH gradients on a weak-base ion-exchange column packing. The ability of these gradients to separate protein mixtures was also investigated. To optimize the conditions used, a computer simulation method using a local equilibrium model developed that predicts the shape of the pH gradient. Several experiments were performed that demonstrate the usefulness of the method and the accuracy of the model.

Gradient chromatofocusing high-performance liquid chromatography. I. Practical aspects

In this work, a versatile method for generating linear pH gradients using weak anion-exchange HPLC has been developed, which is termed gradient chromatofocusing high-performance liquid chromatography. This method utilizes a linear external pH gradient generated in the mobile phase entering the column (inlet pH gradient), superimposed on an internally-generated pH gradient within the column (column pH gradient), which results from the buffering action of the ion exchanger on the mobile phase and vice versa. The method shows significant advantages over conventional chromatofocusing, including: decreased expense due to the use of common buffer components, ease of adjusting the slope of the pH gradient produced at the outlet of the column (outlet pH gradient) through the manipulation of the inlet pH gradient and the ability of using high concentration buffers in the mobile phase. Chromatography of fibrinogen degradation products was done using gradient chromatofocusing. Bandwidths comparable to conventional chromatofocusing were obtained in the separation of fibrinogen degradation products.

pH gradients generated by polyprotic buffers. II. Experimental validation

The experimental validation refers to the computer program reported in the companion paper, able to simulate the course of pH, buffering power (beta) and ionic strength (I) of polyprotic buffers (either singly or in a mixture) titrated over any pH range. With simple oligoamines (up to five nitrogens) it is shown that it is impossible to generate linear pH gradients in the pH 4-10 interval, unless they are mixed in appropriate ratios. With pentaethylene hexamine, when used alone, it is possible to create a linear pH 4-10 interval, provided the molarity ratios are altered in the two chambers of the gradient mixer. The general rule operating for generation of linear pH intervals is constancy of buffering power throughout the titration. Local minima of beta produce steeper gradients, while local beta maxima flatten it. The ideal delta pK to arrange for linear pH gradients during titration is centred around 1 pH unit; thus polyprotic buffers with very large delta pK values (e.g., EDTA) appear to be totally useless for this purpose. The present computing algorithms should be quite efficient for optimizing existing buffer recipes for chromatofocusing or ampholyte displacement chromatography or for creating new, properly tailored, buffer mixtures.

pH gradients generated by polyprotic buffers. I. Theory and computer simulation

This paper presents the general equations for computing pH, dissociation coefficients, buffering power and ionic strength of pure polyelectrolyte solutions (polyacids, polybases and zwitterionic species with any number of dissociable groups) and mixtures of any number of these species. A program has been written for simulating the behaviour of mixtures containing up to 50 species (including buffers and titrants), each of them with up to 10 dissociable groups. This allowed one to check the equations with the available data on a few oligoprotic species.

Proteins associated with untransformed estrogen receptor in vitro. Perturbation of hydrophobic interactions induces alterations in quaternary structure and exposure of the DNA-binding site

Estrogen receptors from calf uteri have been analyzed by high-performance size-exclusion chromatography, chromatofocusing, and DNA affinity chromatography using conditions designed to evaluate the relative contribution of hydrophobic interactions between the steroid-binding subunit and other receptor-associated proteins. The single large (untransformed) species of soluble estrogen-receptor consistently (n = 9) found in calf uteri displayed a rapid change in Stokes radius from 8.0 to 3.5 nm upon exposure to elevated ionic strengths (0.4 M KCl). However, equilibration of the estrogen-receptor complex into urea (up to 6 M) did not dissociate the untransformed receptor into the 3.5-nm receptor form (subunit) observed in hypertonic (0.4 M KCl) buffers. Exposure to 6 M urea did result in conversion of the untransformed receptor (8.0 nm) to a 6.0-6.5-nm receptor form not previously observed in either hypotonic or hypertonic buffers. In the presence of both 6 M urea and 0.4 M KCl, the untransformed estrogen-receptor complex was converted to a smaller receptor form intermediate in apparent size (4.5-5.0 nm) to that observed in 6 M urea or 0.4 M KCl alone. The formation of this 4.5-5.0-nm receptor form was partially estrogen dependent as determined by parallel analyses of unliganded receptor in urea/KCl buffer. The urea-induced change in apparent size (8 nm to 6.0-6.5 nm) at low ionic strength was accompanied by little or no detectable change in net surface charge as determined by chromatofocusing but a complete exposure of the DNA-binding site as evidenced by nearly quantitative interaction with DNA-agarose.(ABSTRACT TRUNCATED AT 250 WORDS)