I've been learning about protein crystallography, and how crystals are made. A lot of the crystallization processes involve gradually changing variables (protein concentration, pH, salt concentration, precipitant concentration etc) for example through vapor diffusion in order to drive the process forward. One of the things I noticed is that it is very rare to see sodium, potassium or chloride ions in the finished crystal structures. The crystals tend to have a lot of water in them (both in channels between the proteins, and bound to the proteins and visible in the crystal structure) but much fewer than expected visible ions. Back of the envelope, 0.15M KCl should have one K+ and one Cl- per 11 nm^3 or per 370 waters; in reality there seems to be almost no ions visible in the structures, except strongly bound ions as cofactors like Ca2+, Mg2+, zinc, iron etc. It is hard to tell if that is because the ions are there but not in a periodic structure, or because they are not there; and also hard to tell what the final concentration of ions is.

Is it possible to crystallize proteins so that the crystal has a concentration of potassium chloride similar to that inside of the cell?

Does having KCl (or NaCl) at normal physiological levels prevent proteins from crystallizing at all? (regardless of other conditions) Or, is this something that is possible but not done usually? (eg in favor of much higher/lower concentrations, or having different ions around) Or, is this already the usual state for prepared crystals? (but just not visible in the structures because the ions are always disordered)

Additional question: If the effective concentration of KCl in crystals is very different from physiological, does this significantly change the protein structure? Has there been any study of the change in structure vs type and concentration of ions?



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