The molecule SDS is an anionic tenside and should dock to the proteins with its lipophylic tail. Basic proteins are charged positively and though the SDS-molecule dock with its negative head - the sulfate group - to the proteins. This will not enhance the negative charges of this mycelles which leads to a bad behavior in SDS-electrophoresis.
Remedy is to change to a TRIS-based buffer-system (ElphoGels) or, the best:
Run a native cathodic electrophoresis (native cathoda buffer kit, DryGel Elpho) Please note that basic proteins will like this applied acidic buffer system....
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