A basic explanation of pI can be found here:

http://www.dilyx.com/protein-solubility-basics/

Take a look here

https://www.researchgate.net/post/Anyone_have_any_advice_on_efficient_purification_of_secreted_His-tagged_proteins_expressed_in_insect_cells_using_Baculovirus_system

and here

https://www.researchgate.net/post/How_to_prevent_protein_aggregation_during_purification

for suggestions on select the correct pH to use for your protein. Ideally you want a pH at least one unit away from the pI (9) to keep the protein charged so electrostatic repulsion keeps the protein from aggregating. Unfortunately, the binding to the His column will be less when His is protonated. You can try a pH around 7 or 6.5 and add glycerol and reduce the salt concentration to keep the protein stable. Some other suggestions (for example, the amino acid arginine can keep some proteins stable can be found in the links.

Finally, remember to consider what assays you will be using after purification.Some of the additives will interfere with particular assays.

http://www.sciencedirect.com/science/article/pii/S0003269703000599

A basic explanation of pI can be found here:

http://www.dilyx.com/protein-solubility-basics/

Take a look here

https://www.researchgate.net/post/Anyone_have_any_advice_on_efficient_purification_of_secreted_His-tagged_proteins_expressed_in_insect_cells_using_Baculovirus_system

and here

https://www.researchgate.net/post/How_to_prevent_protein_aggregation_during_purification

for suggestions on select the correct pH to use for your protein. Ideally you want a pH at least one unit away from the pI (9) to keep the protein charged so electrostatic repulsion keeps the protein from aggregating. Unfortunately, the binding to the His column will be less when His is protonated. You can try a pH around 7 or 6.5 and add glycerol and reduce the salt concentration to keep the protein stable. Some other suggestions (for example, the amino acid arginine can keep some proteins stable can be found in the links.

Finally, remember to consider what assays you will be using after purification.Some of the additives will interfere with particular assays.

http://www.sciencedirect.com/science/article/pii/S0003269703000599

Is the pI theoretical or experimental? I would just go for purifying at pH 7.0 or 8.5 (depends on which ion exchanger resin you want to use). Keep your salt concentration low, if you are nearing the pI.

Dear Amruta

pH at which the net electric charge of aminoacid or protein is zero is called the isoeleetric point or isoelectric pH, designated pI.

The pI reflect the nature of the ionizing R groups of AAs present.

Electrophoresis allows determination of crucial properties of a protein such as its pI and approximate molecular weight. Isoelectric focusing is a procedure used to determine the pI of a protein. Proteins with different pI are thus distributed differently throughout the gel.

Combining isoelectric focusing and SDS electrophoresis sequentially in a process called two dimensional electrophoresis permits the resolution of complex mixtures of proteins.

It is important to know that the buufer that you can use is dependent to other methods that you want to use after purification. If you want to use 2DE after purification, please dont use phosphate buffer that contain more ions. 0.01 M Tris buffer that contain 10% sucrose and have antiprotease cocktail are more suitable.

Cheers

thank you all for the wonderful answers. I appreciate it.

There's a huge body of literature on pI etc as mentioned by other respondents. As for purification via His tag, just use a 20 mM phosphate, 0.5 M NaCl, 20 mM imidazole buffer at pH7.4. We have purified proteins ranging from pI 5 to pI 9 in such buffer with excellent yields and purity.

If any of the components, pH etc interfere with subsequent assays, carry out a simple buffer exchange on PD10 columns or similar.

With a pI near 8 I would try purification at ~pH 7 but remember salt concentration matters too for your His-tag. Why not use the typical 10-25mM imidazole and high salt - 300 mM KCl.

Thank you@Shah

How can I calculate the pH range of a protein from its isoelectric point?