Protein at low PH - Strange behavior of His-tagged protein at low PH (Apr/28/2010 )

Hi all!

I have found strange behavior of my current protein :
a bit about it: His6-tail at N-termini, pI around 6.4 ( theoretical from sequence), Soluble in ordinary buffer: 0.1M Tris pH 8.2, ~250 mM NaCl, 1 mM DTT , 0.01% detergent

But by pH screen I have found that at 0.1M glycine pH 3.0 it solubility raise to 30 mg/ml ( in previous only 10 mg/ml) without any addictive - no salt and even without detergent!
Can someone explain what is happened?

P.S.: If I'll mix protein with conc. 30 mg/ml in 50 mM glycine pH 3.0 with another 100 mM buffer with pH 4.0 protein completely precipitates.

the farther the pH is from the pI the greater the net charge on the protein. at pH 3 your pI 6.4 protein has a strong net positive charge. at pH 8.2 it has a smaller net negative charge.

the low pH may also cause some conformational differences in your protein.

raising the pH with the pH 4 buffer reduces the charge, which may be reducing the solubility.