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I have a question about TBR's CBT 4, question #137.
The passage provides the chemical structure of ADH, which is shown with an overall charge of +2. ADH has 3 dissociable hydrogens, and the pKa's are given:
pKa of N terminus is 8.0
pKa of phenolic hydroxyl is 10.0
pKa of the guanidino group is 12.5
The question asks "What is the isoelectric point (pI) of ADH?"
I thought that the molecule would attain a neutral charge once two hydrogens were removed (via base titration), which in this case would be the two hydrogens with the lowest pKa.
I thought that the pI is then:
pI = (8.0 + 10.0) / 2 = 9.0
The solution says that instead you have to take the average pKa of the second and third hydrogens to determine the pI: (10 + 12.5) / 2 = 11.25
I'm not too confident when it comes to isoelectric point problems, but the solution that TBR provided isn't making a whole lot of sense to me.
Can someone help explain this?
Cheers.
The passage provides the chemical structure of ADH, which is shown with an overall charge of +2. ADH has 3 dissociable hydrogens, and the pKa's are given:
pKa of N terminus is 8.0
pKa of phenolic hydroxyl is 10.0
pKa of the guanidino group is 12.5
The question asks "What is the isoelectric point (pI) of ADH?"
I thought that the molecule would attain a neutral charge once two hydrogens were removed (via base titration), which in this case would be the two hydrogens with the lowest pKa.
I thought that the pI is then:
pI = (8.0 + 10.0) / 2 = 9.0
The solution says that instead you have to take the average pKa of the second and third hydrogens to determine the pI: (10 + 12.5) / 2 = 11.25
I'm not too confident when it comes to isoelectric point problems, but the solution that TBR provided isn't making a whole lot of sense to me.
Can someone help explain this?
Cheers.