Electrophoresis of composite molecular objects. 1. Relation between friction, charge, and ionic strength in free solution

We present a study of the mobility of streptavidin end-labeled ssDNA fragments in free solution capillary electrophoresis. Our data show that the contribution of the uncharged streptavidin label to the net electrophoretic mobility of the complex depends on the buffer ionic strength because this frictional contribution is actually related to the persistence length of the ssDNA fragment. Therefore, the ionic strength of the buffer affects both the free-solution mobility of naked ssDNA and the effective slowing down (or "drag") due to the streptavidin label. We further show that our experimental results can be interpreted using recently published polyelectrolyte electrophoresis theories. Our results thus confirm that the so-called free-draining picture of local ssDNA electrophoretic friction and charge is not valid.