what is the procedure to remove formic acid from partially purified low molecular weight protein solution without using lyophilization?
The pKa of formic acid by is 3.75 (20―25 ºC) and accordingly to the Henderson-Hasselbalch equation, it can be partially neutralized with a strong base, e.g. NaOH, to provide adequate buffering capacity within the 2.75―4.75 pH range. Possibly, the protein you may be working with, has higher isoelectric point, what would help to explain its solubilisation. Being that the case, you may possibly consider to further neutralize the acid, readjusting the pH somewhat below, but yet close enough the isoelectric point. You may possibly consider to then add ammonium sulfate to promote the protein precipitation (salting-out). The protein should by then still have net positive charge, thus offering cationic sites for the sulfate ion to bind. Salts with plurivalent cations (e.g. CaCl2) could possibly be also considered for the same purpose. Addition of a conveniently selected organic solvent to the aqueous solution; such as acetone, ethanol, or methanol ― non-solvent for the protein ― could also be considered. It may help to add the solvent cold to previously thawed or warmed aqueous solution. The precipitated protein could then be filtered-off or centrifugated out from its mother solution. Henceforth, it could be washed with (possibly cold) water, or with another compatible solvent, to remove any remaining formic acid and/or salt(s).
Alternative methods to purify proteins from accompanying soluble acids or salts, can, in principle, involve its precipitation by ultracentrifugation (possibly followed by washing the sediment), liquid-liquid extraction, ultrafiltration, suitable chromatographic methods, ion-exchange, electrophoresis, dialysis; among other methods that could be feasible for the protein considered.
Elsewhere at this forum, I have discussed the prediction of pH for aqueous solutions of formic acid or for its solutions with formate salts ― taken as strong electrolytes ― without making use of the Henderson-Hasselbalch equation: https://www.researchgate.net/post/pH_calculation_of_a_mixture_of_formic_acid_NaOH_and_water
Using Desalting Column
https://www.thermofisher.com/kr/ko/home/life-science/protein-biology/protein-biology-learning-center/protein-biology-resource-library/pierce-protein-methods/overview-buffer-exchange.html
https://www.bio-rad.com/featured/en/desalting-column.html
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