A novel interpretation of steady-state electrophoresis experiments

In steady-state electrophoresis the macroion concentration c(x) is shown to be proportional to exp(nu(el) x/D) at low values of c(x), where nu(el) is the electrophoretic velocity and D is Fick's diffusion coefficient of the macroions in free solution. For 20 bp DNA, studied by Laue et al. (J. Pharm. Sci. 1996, 95, 1331) in a novel membrane constrained electrophoresis cell, the steady-state concentration pattern is used to test the values of nu(el) and D determined under the same conditions. If the macroion/salt ratio is not low, the c(x) pattern is biased by interactions between the macroions and by the conservation of current requirement. The calculation of an effective charge of macroions, Q(eff), from electrophoretic experiments is discouraged because Q(eff) is not a useful molecular parameter for several reasons. (a) Q(eff) is not simply related to the structural charge of the macroion. (b) For macro-ions with high surface potential Q(eff) depends also on the mobility of the small ions. (c) Q(eff) may depend on the type of experiment. (d) For nonspherical macroions Q(eff) depends on orientation.