High-resolution computer simulation of the dynamics of isoelectric focusing using carrier ampholytes: the post-separation stabilizing phase revisited.

A dynamic electrophoresis simulator that accepts 150 components and voltage gradients employed in the laboratory was used to provide a detailed description of the stabilizing phase in isoelectric focusing under conditions that were hitherto inaccessible. High-resolution focusing data are presented for pH gradients spanning 7 units (pH 3-10 and pH 4-11 with 20 carrier ampholytes/pH unit) and 3.5 units (pH 7-10.5 and pH 5-8.5 with 40 carrier ampholytes/pH unit). Stabilizing phase behavior for configurations (i) with the focusing column ends only permeable to OH(-) and H(+) at cathode and anode, respectively, and (ii) with the focusing column being sandwiched between NaOH (catholyte) and phosphoric acid (anolyte) are described. Simulation data reveal the stabilizing phase to be diffusion-controlled and characterized by changes that progress from the column ends towards neutrality (i.e., towards the center in case of pH gradients bracketing neutrality). Transient states are characterized by moving concentration valleys of carrier ampholytes that significantly alter the distributions of pH and conductivity. Nonlinear pH gradients are produced. The magnitude of the changes occuring is dependent on the span of the pH gradient. Gradients that encompass greater extremes of pH show more pronounced stabilizing phases. For all systems subjected to a constant 300 V/cm, the initial separation and subsequent stabilization require less than 10 min and more than 7000 min, respectively. The presence of electrolytes at the column ends disrupts the stabilizing phase, with the degree of disruption dependent on the concentrations of the acid and base employed as electrode solutions. The data not only indicate that a true steady state is never attained in the average laboratory experiment, they also suggest that a true steady state in absence of immobilized pH gradients cannot be achieved experimentally at all.