shift of protein bands in SDS gel electrophoresis - where does it come from, can it be improved (Aug/22/2005 )

Hi,
presently I study the phosphorylation of a peptide which shows a shift to higher molecular mass when phosphorylated. How is this shift explained? Less binding of SDS because of the negative charge of the phosphate group? Has anyone experience how this shift can be improved to get well separated bands? I tried gradient gels, but this did not help.
Thanks

I am not sure what causes the shift either, but you might get better separation using a urea gel.

Thank you, I will try urea gels