And if so, how low does the pH need to be to elute cleanly in a small volume?
pH can be run as a gradient as easily as a salt, or imidazole, can. Run a gradient, collect fractions and assay for your protein. Normally imidazole, which structurally resembles the ring on histidine, competes with the histidine interactions with the Nickel on the column. At low pH, the histidine ring becomes protonated and your His-tagged protein comes off. I think this happens below pH of about 7, but not sure. But the answer is related to the pKa of the histidine's R-group.
I know it's possible but never tried,
according to GEH:
http://www.gelifesciences.com/gehcls_images/GELS/Related%20Content/Files/1336168762999/litdoc18114275_20150126213740.pdf
Page 31:
Alternative elution solutions
As alternatives to imidazole elution, histidine-tagged proteins can be eluted by other methods or combinations of methods; for example, lowering of pH within the range of 2.5 to 7.5 can be used. Below pH 4, metal ions will be stripped off the chromatography medium.
It is not always possible to elute with lower pH when using a metal ion other than Ni2+. This is protein and metal ion dependent.
EGTA and EDTA, which are strong chelating compounds, can also be used for elution, but they will strip the metal ions from the medium and thereby cause protein elution. The co-eluted metal ions will remain chelated in the protein solution, but are easily removed with a desalting column (see Chapter 11).
Yes. It is very much possible and we have done pH based elution. Usually we equilibrate the Ni-column with higher pH (8.0 or so) and bind the His tag-protein lysate at same pH buffer and wash with little lower pH (~ 7.0) buffer. Then you can elute the bound protein at lower pH (5.0) buffer. It works based on Histidine chemistry ( protonation and de-protonation of the pKa) which is important to form complex with the Ni+2 metal.
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