Hi all,
I want to understand the heat changes that come from anions (PO4-) interacting with metal ions (Cu^2+). PO4- is supposed to interact with metal ions very strongly as it is used to precipitate divalent ions. Would this interaction significant enough to show an obvious change of heat?
For example, let say, we want to observe the thermodynamics of metal binding to a certain protein. But the solution contains :metal binding protein + (Cu^2+) + and PBS buffer (PO4- buffer;137mM NaCl and 10mM phosphate). In this case, phosphate will interact with the divalent metal ions.
Would the interaction between the phosphate and metal ions contribute to the changes in enthalpy observed in ITC?
Copper (II) phosphate is insoluble in water, so this experiment would fail. You need to use a buffer that has no copper binding affinity. According to this paper (http://pubs.acs.org/doi/abs/10.1021/ac0261101?journalCode=ancham), MES and MOPS do not bind copper.
Yes. It's actually someone else's experiment that I am trying to investigate further... They already performed ITC experiment and calculated Kd. They said that the binding of copper to the protein is in pM range so I think the binding may outweigh the interaction with solvent.. But I'm just trying to see what would be the change in the heat for copper interacting with phosphate buffer.
This is a very complex question which is probably can be answered. You need to collaborate with an expert with deep knowledge of chemical thermodynamics. I'm really busy with several projects to help you.
Dear Adam,
you wrote that "Copper (II) phosphate is insoluble in water." This is not absolutely true, it depends on concentrations, pH, ionic strength , etc. Cu(II) is pretty well soluble in diluted phosphate buffer at neutral pHs..
I found a reference that gives the solubility product (Ksp) of copper (II) phosphate as 1.4 x 10-37 at 25oC. That's pretty insoluble! I assume that is for distilled water, so adjusting the pH could have a big effect.
When doing ITC experiments, it is customary to do a blank run, injecting the buffer instead of the ligand. The heat from the blank run is then subtracted from the complete run. If that was done in the experiment in question, the solvent interaction should already have been dealt with.
The molecular fraction of PO4(3-) in phosphate buffer at pH 7.2 is 0.0000035. The solubility products K(sp) = [Cu]^3 x [PO4(3-)]^2. The solubility of Cu in 10 mM phosphate buffer [Cu]^3 x [PO4(3-)]^2 = [Cu]^3 x [PO4(3-)]^2 = [Cu]^3 x (1e-4) x (1e-15) = 1.4 x (1e-37). This gives [Cu] ~ 1e-6 M. However, this calculations does not make much sense since Ksp=1.4e-37 of copper (II) phosphate is often cited, but not accepted as reliable as no account was taken of the complex formation between Cu2+ and HPO4
2–. This is a simple example how complex is the system.
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