How is the isoelectric point for an amino acid or peptide mathematically defined? Does it require that the solution be buffered to the pI?
I raise this concern because the charge balance must be invalid here - I have seen pI defined as pI=1/2(pKa1+pKa2) for some amino acids, but then we find that at pI, [H2A+]=[A-] (the protonated form of the acid is equal in concentration to the completely deprotonated form of the acid, for some only) - so using this in the charge balance means [H+]=[OH-] unless the charge balance must be dropped for some reason.
My guess is, this means that 1) the amino acid, alone in solution, can never be brought to the isoelectric point, regardless of dilution (this in fact suggests to me that the pH of the pure amino acid solution is independent of initial concentration), unless pI=7 for that acid; 2) to reach pI, we are adding other ions to the amino acid solution to change the pH, and so without knowing their identity we can't use the charge balance.
I raise this concern because the charge balance must be invalid here - I have seen pI defined as pI=1/2(pKa1+pKa2) for some amino acids, but then we find that at pI, [H2A+]=[A-] (the protonated form of the acid is equal in concentration to the completely deprotonated form of the acid, for some only) - so using this in the charge balance means [H+]=[OH-] unless the charge balance must be dropped for some reason.
My guess is, this means that 1) the amino acid, alone in solution, can never be brought to the isoelectric point, regardless of dilution (this in fact suggests to me that the pH of the pure amino acid solution is independent of initial concentration), unless pI=7 for that acid; 2) to reach pI, we are adding other ions to the amino acid solution to change the pH, and so without knowing their identity we can't use the charge balance.
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