I want to measure different ions (i.g. Ca2+, Mg2+, Zn2+) in a complex proteins mixture with the MS. However, in the protein mixture some ions will most likely be bound to proteins. I want to free the ions from the proteins, so I can measure them. What is the best way to do this? I was thinking about some kind of method that destroys the proteins but does not precipitate them. Does such a method exist? If so, what is the protocol for this method? Since I'm only interested in the ions, it does not matter what the effect of the method on the rest of the sample is.
I believe the standard here is still atomic absorption spectroscopy.
The electroneutrality constraint rules. Whatever you do to get rid of the protein, you will have to provide counter ions to maintain the ions in solution, otherwise they will just come out with the proteins. You can try to chelate the divalent cations with EDTA and EGTA, which might fly as adducts, but even as chelates there must be a negative ion around to balance the charge, so little will fly. Same is true with polyethers (e.g., PEG), which will form adducts with Na and K ions. You best bet is to use a volatile organic as the negative counter ion (e.g., acetate or formate) as some of this will suck up a proton to volatilize.
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