The electrophoretic mobility of a molecule in an electric field in free solution depends on its total charge density. Its measurement can therefore potentially yield the number of charged moities comprising a biological macromolecule. However, at high charge densities counterion condensation can occur: strongly bound counterions effectively mask some of the biopolymeric charge and complicate the relationship between mobilities and the number of charged residues. While current predictions of counterion condensation are based on a molecule-averaged charge density, for many biological macromolecules this approximation is not appropriate.
Experiments have been carried out using a simple linear anionic polysaccharide as a model sysetm in which the sugar residues can be charged or not (methylesterified). Crucially the amount and distribution of charged residues can be modified. In this study a comparison of the electrophoretic mobilty of these anionic polysaccharides with different charge distributions but the same overall charge was carried out using capillary electrophoresis. It was found that for polymers in which charges were distributed in “blocks” electrophoretic mobilities were characteristic of nominally lower degrees of charged residues than was observed for polymers with random charge distributions. This suggests that counterion condensation is also occuring for high local charge densities.
By applying the model for counterion condensation over sub-sections of the molecule and subsequently summing the derived “local charges” associated with these lengths to obtain a molecular charge density, and comparing these with the measurements of electrophoretic mobility we estimate the lengthscale over which counterion condensation must be considered.