The Relationship between Electrophoretic Mobility and Polyelectrolyte Charge

Abstract

Many textbooks state that the electrophoretic mobility of a polyelectrolyte is proportional to its effective charge divided by its frictional coefficient. We have tested this relationship by analyzing the mobilities of single- and double-stranded DNA molecules containing the same number of bases or base pairs and different numbers of negatively charged phosphate linkers. Small organic molecules containing different numbers of charged residues were also analyzed,using data taken from the literature. In each case, the free solution mobilities of the charge variants of a given molecule were divided by the mobility of the charge variant with the highest number of charged residues, measured under the same conditions. The mobility ratios were then plotted as a function of the fractional charge of each molecule. The results indicate that the fractional mobilities of polyelectrolytes of the same size are proportional to the logarithm of the fractional charge, not the first power of the charge as commonly assumed. The Manning theory of DNA electrophoresis and electrophoretic theories based on the zeta potential both predict a logarithmic dependence of the mobility on charge density. The experimental mobility ratios will be compared with the predictions of these two theories.