Why is there extra band in IMAC SDS-PAGE - (Feb/25/2010 )
I am running at SDS PAGE on a His-tag purified protein at around 14kDa.
I ran the SDS-PAGE 3 x and each of them have an extra band at the 20kDa.
The SEC peak show only 1 peak and the weight calculated not to be twice as much, so most probable it is not a dimer.
What could be in the 20kDa band? The crude E.coli lysate also display this band.
Can anyone advise??
Perhaps you can gel run the cell lysate w/o your plasmid to act as a negative CTRL. This will at least tell you whether that extra band is due to your protein or some other protein from the cells.
Note: many people have reported formation of dimers upon boiling before loading, although your new band is bit lighter to be a dimer.