Isoelectric focusing using non-amphoteric buffers in free solution: III. Separation of amino acids

Separation and fractionation of glutamic acid and histidine via origami isoelectric focusing

We demonstrated the fractionation of two amino acids, glutamic acid and histidine, separated via isoelectric focusing (IEF) on filter paper folded and stacked in an origami fashion. Channels for electrophoresis were fabricated as circular zones acquired via wax printing onto the filter paper. An ampholyte solution with amphiphilic samples was deposited on all the circle zones, which was followed by folding to form the electrophoresis channels. IEF was achieved by applying an electrical potential between the anodic and cathodic chambers filled with phosphoric acid and sodium hydroxide solutions, respectively. A pH gradient was formed using either a wide-range ampholyte with a pH of 3 to 10 or a narrow-range version with a pH of 5 to 8, which was confirmed by adding pH indicators to each layer. The origami IEF was used to separate the amino acids, glutamic acid and histidine, by mixing with the ampholytes, which were deposited on the layers. The components in each layer were extracted with water and measured by high-performance liquid chromatography using pre-column derivatization with dansyl chloride. The results indicated that the focus for glutamic acid and that for histidine were at different layers, according to their isoelectric points. The origami isoelectric focusing achieved the fractionation of amino acids in less than 3 min using voltage as low as 30 V.

Characterization of immobiline membranes for application in a multicompartment electrolyzer for protein purification

The efficient use of preparative protein purification in a multicompartment electrolyzer with Immobiline membranes depends on the knowledge of membrane characteristics. For that purpose, an experimental investigation of the effects of ionic charges on the membrane characteristics has been carried out through the measurements of membrane swelling and conductance. We also investigated the effects on the electrolyzer behaviour of operating parameters such as the Immobiline concentration and the presence of ion-exchange membranes. Data show that polyacrylamide gel degree of swelling is strongly dependent upon the pH and the ionic strength of the bathing solution as well as on the type and molarity of charges incorporated in the gel. The conductance of supported Immobiline gels in contact with uni-univalent chloride solutions has been measured by means of a mercury cell. The membrane conductance is also influenced by the ionic strength of the equilibrium solution and the presence of weak ionizable groups in the gel matrix. This study has demonstrated the close link between electrochemical and electromechanical properties of Immobiline membranes.

Amphoteric, isoelectric immobiline membranes for preparative isoelectric focusing

Amphoteric, isoelectric agarose membranes, as devised by Martin and Hampson [Martin, A.J.P. and Hampson, F. (1978) J. Chromatogr. 159, 101-110], are found unsuitable for blocking electroendosmosis in multi-compartment electrolysers during preparative isoelectric focusing, due to the poor and highly unpredictable incorporation of carboxyls and amino groups on the polysaccharide moiety. New, polyacrylamide-based membranes are described, containing as buffers and titrants the Immobiline chemicals used to produce immobilized pH gradients. These new membranes are supported on both faces by a non-woven polypropylene cloth, a material exhibiting minimal adsorption properties for proteins. Due to the extensively developed Immobiline technology, membranes with highly predictable isoelectric points, well-defined buffering capacity and conductivity can be synthesized at any pH value along the pH 3-10 scale. They are effective in blocking electroendosmosis even when the delta pH on either side of the membrane is as high as 1.5 pH unit.

Isoelectric focusing using non-amphoteric buffers in free solution: II. Apparatus and measures of pH stability

Isoelectric focusing of amino acids or proteins requires a time-invariant pH gradient. The maintenance, in an electric field, of such profiles with simple non-amphoteric buffers is possible in multicompartment cells using well determined concentration profiles. The method for determining these concentration profiles has been proposed in a preceding paper. A multicompartment cell has been built for the purpose of verifying the assumptions made. The technical characteristics of this cell will be described here. The time-stability of pH profiles obtained with sodium acetate/acetic acid buffers has been measured for concentration profiles determined so as to keep constant the transport number of each ion throughout the cell. Measured over a time span of 10 h and with a current density of 96 mA cm-2, the pH shift is, in the worst case, 0.009 pH unit/h. This corresponds to a much better stability than the one obtained with a constant concentration of sodium acetate in all compartments, and justifies the chosen concentration profiles based on the condition of constant transport numbers. The above mentioned method for the computation of the buffer concentration assumed a constant relative mobility of the ions. The experiments have shown the limits of this assumption, i.e. the variation of ionic mobilities with the ionic strength and the temperature diminishes the stability of the pH gradient. However, slight adjustments of the concentration in the end compartments allow some improvement of the stability. If the temperature could be precisely controlled in each compartment, a better pH stability than what has been achieved in these experiments should be reached. Two methods of compensation of the migration of ions in the end compartments (buffer renewal method and external recycling method) have been tested and will be discussed.