# Why does a protein crystallize with a specific salt but not with another?

So I am conducting an experiment and I do not understand this super fundamental part. I know salting out works via solubility, and I know crystallization works via ordered structures. But could someone explain to my why one salt allows a crystal to form, but another salt doesn't. A component of one of my salts (Mg) has been found to be important to my protein in optimization and activity, but why would another cation with the same anion (I know the anion is not the problem) discourage the formation of crystals. If this is a clearly stupid question I apologize.

EDIT:looking for theory. I have already crystallized everything, and have CRYSTALS. Will be performing x-ray crystallography.This post was more to get me on the right track in thinking.

• I'm not getting it, clarify your question. – Mithoron Nov 10 '17 at 22:51
• why would a salt, such as NaCl allow crystallization with a protein, but another salt like CaCl have no crystal formed. If both are salts playing on the solubility of the protein, and interacting more or less the same, what is the difference. @Mithoron – juliodesa Nov 10 '17 at 22:55
• Precipitation isn't equal crystallisation. Good crystals of proteins are often very difficult to make. There may be specific number of Mg cations usually bound to specific protein, but it has not much of a connection with what happens if solution make the protein bind much more of them. I think you should read much more about this. en.wikipedia.org/wiki/Protein_precipitation may be a start. – Mithoron Nov 10 '17 at 23:14
• I understand this; however, I have already formed the crystals and photographed them and will be using them in xray crystallography this weekend. They are not just in phase change or precipitates, but rather beautiful large crystals. My question was more on theory, as to why one of my plates has crystals using one salt at different concentrations and the other (with a different salt) has nothing – juliodesa Nov 10 '17 at 23:52
• sorry I was trying to be as vague as possible intentionally to understand this amongst all salts but okay I am using 3 chloride salts: NaCl, MgCl2 and CaCl2 and three sulfate salts: NaSO4, MgSO4, and CaSO4. But you did just make me realize I needed to consider the pH of the SALTS on the protein(and not just my buffers which were sodium phosphate, and acetic acid), I don't know why that escaped my mind;however still fails to explain why one nacl and mgcl2(both at pH 7 have two different effects) And yes I realize now I should have thought more about this before... @Mithoron – juliodesa Nov 11 '17 at 0:18

We don’t know and nobody knows why a $\pu{1.3M}$ solution of magnesium chloride might help the crystallisation but a $\pu{1.4M}$ or a $\pu{1.5M}$ solution don’t.
We don’t know and nobody knows why $\pu{23^\circ C}$ might be a good temperature for crystallisation but $\pu{21^\circ C}$ and $\pu{25^\circ C}$ aren’t.