About pH and pI for his-tagged protein purification - (Jun/24/2005 )

Hi all,
I have a question about pH and pI.
My fusion protein have pI 6.92 by threoretical, and I have buffers (pH 7.5).
Should I lift up my buffer pH ?
Or, how much the different between buffer pH and pI of the protein should be ?
Thank you very much for your responses.

me also, i am very interested in question, because I should purify the protein of pI 8.3 , and by buffers are pH 8.0. I posed a question on this forum, but nobody replied.

One I have heared from profys, that the parabole, which is forming when you put pH on X axis, and solubility on Y axis is quite different for every protein. For one it is thin enough, so even small difference among pH and pI give quite big solubility, and for that whose parabole is thick, difference among pH and pI must be bigger.

But how to determine before big and laborious production, which pH should be used???

For binding his-tagged proteins to the Ni-affinity resin a range of pH from 7.0 to 8.0 is OK. Select the pH value wich is at least one pH unit away from the pI of your protein.