The distinct advantage of acrylamide gel systems is that the initial concentrations of
acrylamide and BIS control the hardness and degree of crosslinking of the gel. The
hardness of a gel in turn controls the friction that macromolecules experience as they
move through the gel in an electric field, and therefore affects the resolution of the
components to be separated. Hard gels (12-20% acrylamide) retard the migration of
large molecules more than they do small ones. In certain cases, high concentration
acrylamide gels are so tight that they exclude large molecules from entering the gel but
allow the migration and resolution of low molecular weight components of a complex
mixture. Alternatively, in a loose gel (4-8% acrylamide), high molecular weight
molecules migrate much farther down the gel and, in some instances, can move right
out of the matrix.