What is the best way to separte via isoelectric points?

[MCAT guy]

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What is the BEST pH range for buffering a gel to be used to separate cysteine from histidine by gel electrophoresis?

• 5.1 - 7.6
• 7.6 - 9.2

I originally answered the 7.6-9.6 answer, reasoning that I would have one in the range of it's isoelectric point and the other in the range outside of it's isoelectric point. But the answer was to have them both in the range of their isoelectric points. Wouldn't the 7.6 pH also work, if not better?

Solution:

The pH of the solution should lie between the isoelectric points of cysteine and histidine, so that one carries a partial positive charge (and thus migrates toward the cathode), and the other carries a partial negative charge (and thus migrates toward the anode). The isoelectric point of cysteine is 5.1, The isoelectric point of histidine is 7.6, The best pH range for the separation of histidine and cysteine is any pH value between 5.1 and 7.6.

 

[loveoforganic]

At high pH (at least until cysteine is deprotonated), they're both going to be -1 and thus move through the gel at the same rate.