Electrophoretic mobilities of neutral analytes and markers in aqueous solutions of Hofmeister salts

Small neutral organic compounds have traditionally the role of markers in electrophoresis, as they are expected to have zero electrophoretic mobility in external electric fields. The background electrolyte contains, however, ions which have unequal affinities to the neutral molecules, which in turn results in their mobilization. In this study we focused on two electrophoretic markers thiourea and dimethylsulfoxide, as well as on N-methyl acetamide as a model of the peptide bond. By means of capillary electrophoresis and all atom molecular dynamics simulations we explored mobilization of these neutral compounds in large set of Hofmeister salts. Employing a statistical mechanics approach, we were able to reproduce by simulations the experimental electrophoretic mobility coefficients. We also established the role of the chemical composition of marker and the background electrolyte on the measured electrophoretic mobility coefficient. For N-methyl acetamide, we interpreted the results in terms of the relative affinities of cations vs. anions to the peptide bond.