200 mg/ml and you have a protein solution? That is hard to believe. Anyway, something is missing when running the dilution if a precipitate is produced.

What is the buffer?

Perhaps, pH, ionic strength, additional polymers are required to keep the protein in solution.

Falk Fish The buffer is 20mM CA, 100mM Arg-HCl and 0.02% Tween 20. I tried many different buffer conditions, including changing the pH , salt concentration, and adding the sugar, but the protein still precipitates during dilution.

What is CA? If it is Calcium, I suggest to remove it and add a chelator (e.g. EDTA).

Anyway, perhaps unrelated protein or another polymer are needed as carriers (e.g Ficoll, Albumin), anti-oxidants (2ME, glutathione), solubility promoters (Magnesium ion).

At 200mg/mL the protein protects itself...

I found this paper and suspect there are others. IgG's are common research proteins, so someone has likely found a good formulation buffer. It sounds like you are trying (and Falk Fish is suggesting) the right kinds of things.

If you solve you problem, be sure to post the solution in this thread to help people in the future. Good luck!

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