  We investigate the behavior of single polyelectrolytes in multivalent salt
solutions under the action of electric fields through computer simulations. The
chain is unfolded in a strong electric field and aligned parallel to the field
direction, and the chain size shows a sigmoidal transition. The unfolding
electric field $E^*$ depends on the salt concentration and scales as $V^{-1/2}$
with $V$ being the ellipsoidal volume occupied by the chain. The magnitude of
the electrophoretic mobility of chain drastically increases during the
unfolding. The fact that $E^*$ depends on the chain length provides a plausible
mechanism to separate long charged homopolymers by size in free solution
electrophoresis via the unfolding transition of globule polyelectrolytes
condensed by multivalent salt.
