I recently had to separate a 71, a 68.2 and a 68.7 kDa protein. I think
7% works the best. I tried 7.5% and 6.5%. But to get a good separation I
had to run a long gel. I used a 30 cm sequencing gel with 1.5 mm spacer and
got a good separation. I was able to distinguish between the band of 68244
and 68726 kDa as determined from the sequence. But the bands diffused and
were not as nice as on a short fast gel. It might be because I ran it
overnight ? If I had to do it again I would use spacer of 1 mm or even
less. It should reduce the diffusion.
I should mention that I transfered a part of the gel to Nitrocellulose and
detected my bands by Western Blotting.