Chromatography Forum

Chris, thank you again for taking the trouble to answer this question.

Everyone agrees that ammonium sulfate is a good protein precipitant. Sulfate is strongly hydrated and thus should tie up enough water to render the protein insufficiently solvated to stay in solution. However, the ammonium ion is relatively weakly hydrated according to my data, and less hydrated for example than the sodium ion. In this case, sodium sulfate should be a better protein precipitant, but it is not. Can you explain this? I am not sure about the TCA example that you gave. I thought TCA denatured proteins and caused their precipitation-one of the reasons for this is the low pH of TCA. Is this not different from the case of precipitation without denaturation as with ammonium sulfate? If I remember, the original 1890 Hofmeister series did not take pH effects into account, but the "amended" version does.

Chris- I would be extremely grateful if you could also answer my question on the point you raised about WAX chromatograpy. You said that highly hydrated ions have less retention. Can you explain this more fully? Are you suggesting this is some sort of size exclusion effect where the hydrated ions cannot enter the pores of the resin? Or is there some other reason like the increased distance between point charges caused by the hydration of the solute ion and its effect on Coulombic attraction?

Victor,
the info on such series is somewhat confusing, that is, different authors give different sequences. It probably has something to do with how the data were obtained. For instance Eriksson in J-C Janson and L Ryden, Protein Purification, VCH, NY, 1989, page 220 gives this series for strength of interaction of a protein with a HIC gel (decreasing salting out ability): Na2SO4>NaCl>(NH4)2SO4>NH4Cl>NaBr>NaSCN.
Maybe ammonium sulfate is on the top of the list of some authors because they had a pH effect in addition? Sometimes one gets the distinct feeling that a "scientific pope" forces some erroneous results on the rest of the community.
Here is an interesting link related to this:

http://bama.ua.edu/~rdrogers/webdocs/ABS/tsld020.htm

On this denaturing: It seems that all of these modifications, pH, salts, detergents, organic solvents can denature proteins. The mechanisms appear somewhat different, for instance, the disruption of a hydrogen bond due to a proton is different than one caused by changing the polarity of water via a salt. Now I have to pass in explaining this in detail, it is complex to say the least....it´s life! (The main reason that I am passing here is that I don´t really understand why with anions the increase in free energy of hydration runs parallel to an increase in order of the solution, while with cations the opposite seems to hold).