Physico-chemical properties of amphoteric, isoelectric, macroreticulate buffers

Amphoteric, buffering chromatographic beads for proteome prefractionation. I: Theoretical model

The possibility is reported here of fractionating proteins on amphoteric, buffering resins via ion-exchange chromatography. A given protein's adsorption to a particular amphoteric buffering resin is characterized by a bell-shaped curve in which the maximum protein binding capacity is observed at an optimum pH value lying approximately midway between the isoelectric point values (pI) of the resin and the protein. On either side of this maximum the protein binding capacity declines steadily, reaching zero at the pI of either the protein or exchanger. For instance, on beads of pI equal to 8, four proteins, two acidic (bovine albumin and ovalbumin) and two basic (cytochrome c and lysozyme), exhibit binding curves reaching zero values for the whole set when the exchanger is conditioned at pH 8.0. Away from the pI, and on both sides of the pH scale, the bell-shaped adsorption curves reach a maximum, for each protein, at a pH located at the midpoint between the pI values of each protein and that of the exchanger, and decline steadily to reach zero at the pI value of each protein species. Separation of model proteins using different amphoteric buffering resins of various pI was possible at different pH values according to both the pI of the proteins and of the exchangers. It was also demonstrated, using surface enhanced laser desorption/ionization mass spectrometry and two dimensional electrophoretic mapping, that separation of an Escherichia coli cell lysate on columns packed with amphoteric buffering resins of different pI and titrated to a particular pH value, delivered two distinctly different fractions, i.e. characteristically composed of, on the one hand, proteins having a pI below the buffer pH (the 'adsorbed' fraction), and on the other, of alkaline proteins possessing a pI above the pH of the buffer (the 'unadsorbed' fraction). This approach represents an attractive addition and/or alternative to the armory of protein pre-fractionation techniques currently employed in proteomics.

Separation of proteins in a multicompartment electrolyzer with chambers defined by a bed of gel beads

Multicompartment electrolyzers (MEs) with isoelectric membranes were introduced in 1989 for purifying proteins in an electric field. At the basis of ME technology there are membranes consisting of cross-linked copolymers of acrylamide and acrylamido monomers bearing protolytic groups. The technology employed for casting the membranes is an extension of the isoelectric focusing in immobilized pH gradient technique for which specific acrylamido monomers, known with the trade name of Immobiline, have been developed. However, the use of continuous membranes presents several disadvantages. Due to the mechanical characteristics of polyacrylamide, the gel must physically adhere onto a rigid support, which prevents it from collapsing. The support must have a highly porous structure in order to be permeable to proteins. The mechanical fragility of the membranes is one of the main problems that hinders the industrial scale application of ME separators. In order to overcome this problem, we propose to substitute the continuous membranes with a bed of gel beads of identical comonomer composition, obtained by an inverse emulsion polymerization process.

Two-dimensional electrophoresis of basic proteins with equilibrium isoelectric focusing in carrier ampholyte-pH gradients

A modified procedure for the two-dimensional electrophoretic analysis of basic polypeptides is described. This method uses isoelectric focusing with carrier ampholytes in the first dimension, and sodium dodecyl sulfate-electrophoresis in the second dimension. Counteraction of the cathodic drift is achieved by glass tube treatment (silanization), electrolyte modification (use of weak bases and acids), protection of the catholyte from carbon dioxide, and the addition of glycerol to the gel mix. Better resolution and reproducibility are obtained than with nonequilibrium pH gradient electrophoresis, since quasi equilibrium focusing can be obtained.

Towards new formulations for polyacrylamide matrices: N-acryloylaminoethoxyethanol, a novel monomer combining high hydrophilicity with extreme hydrolytic stability

Matrices for electrokinetic separations, based on a unique class of mono- and disubstituted (on the amido nitrogen) acrylamides such as e.g., N-acryloylaminoethoxyethanol (AAEE) and acrylamido-N,N-diethoxyethanol, offer the following advantages: (i) strong resistance to alkaline hydrolysis (most zone separations occurring at basic pH values), (ii) high hydrophilicity and (iii) greater porosity, due to the higher molecular weight of the monomers. When compared with conventional poly(acrylamide), a poly(AAEE) matrix, when subjected to mild alkaline hydrolysis (0.1 N NaOH, 70 degrees C) appears to be 500 times more stable. Such stability is also confirmed under strong alkaline hydrolysis (1 N NaOH, 100 degrees C) as well as under mild and strong acidic hydrolysis. Mildly hydrolyzed poly(AAEE) matrices still perform extremely well in both conventional isoelectric focusing and immobilized pH gradients, techniques which are quite sensitive to traces of acrylate in the polymer coil. Conversely, mildly hydrolyzed poly(acrylamide) matrices, when used in isoelectric focusing, generate pH gradients between pH 4 and 5, having an inflection point (pH 4.6) equivalent to the pK value of acrylic acid. This novel class of monomers shows great promise for future applications in all electrokinetic methodologies.