Predicted vs actual MW

Nick Theodorakis nicholas_theodorakis at
Sun Nov 7 22:58:25 EST 1999

In article <pxpst2+-0611991043130001 at>,
pxpst2+ at (Peter) wrote:
> In article <3821AEEE.9EFC3D58 at>, Lee Hunt
> <L.hunt at> wrote:
> > Does anybody have explanations as to why a protein on a Western
> can
> > differ widely from its predicted size. Other than proteolysis or
> removal
> > of targetting regions what else could cause this?
> Always remember that PAGE does not seperate purely by mass/size.
> Seperation is by the charge to mass (m/z).  Alter this and and you
> will
> have abherent migration.
> Regards,
> Peter Pediaditakis

That's the textbook version for "pure" electrophoresis, anyway. In real
life, (for biochemists/molecular biologists, anyway), molecular sieving
is important. Otherwise, all DNA should have the same electrophoretic
mobility, and to a first approximation, all SDS-covered proteins should
too. Actually, all DNA does have the same m/z and have the same
electrophoretic mobility and would run the same place, except that
those pesky agarose or polyacrylamide strands get in the way.


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