Hey scientists,
I am trying to drug a protein which is soluble between a pH of 5.0 to 9.0. My goal is to crystallize an amine drug with this protein and I know optimal conditions that crystallize my protein at slightly acidic conditions at pH 5.5. However, the protein is just a precipitated mess at lower pH. My drug is a pure hydrophobic liquid. Logically, addition of HCl converts this amine drug to the ammonium salt which is soluble at about 150 mM which is really impressive as I have observed.
That means that I put 150 mM of my amine drug in solution and I need 150 mM of HCl in solution to convert my drug to the soluble ammonium salt. However, now the drug is soluble, but the pH of the solution is less than 2. If I put double the concentration (300 mM) of a citrate pH 5.5 solution, that raises the pH to about 4 as estimated with a pH strip. But when I add my ultrapure protein that I just purified today, the protein precipitates.
I know it's the HCl that is precipitating my protein because ...
protein + H2O + high concentration citrate buffer ---> soluble protein
protein + H2O + HCl --> insoluble protein
protein + H2O + citrate buffer + HCl --> insoluble protein
Basically, HCl makes my amine drug very soluble, but the acidity makes my protein insoluble.
The drug isn't soluble in acidic conditions unless I add HCl to convert the drug to the ammonium salt.
If I added my amine drug to solution + HCl to solubilize the drug, I should be able to recover the ammonium salt, right? Then I fast evaporate the solution to remove the HCl and recover a couple milligrams of the ammonium salt, right? Is that the best solution?
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