TBR Test 4: finding pI

[Chocolatebear89]

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Vasopressin has 3 dissociable hydrogens. One is on the N-terminus (pKa ≈ 8.0), one is on the phenolic hydroxyl (pKa ≈ 10.0), and the last is on the guanidino group of arginine (pKa ≈ 12.5).What is the isoelectric point (pI) of ADH?

 

[Rabolisk]

The pI is about 11.3. Arginine is a basic amino acid. Thus the most protonated form of vasopressin has two positive charges. The first pKa thus corresponds to the first positive charge, and the second pKa is the second positive charge. pI is the point at which you have a neural molecule, which is roughly the average of the pKa that corresponds to the last positive charge, and the pKa that give a singly negative charged molecule. That might have been one of the more convoluted explanations yet.

 

[Prateek]

The pI is about 11.3. Arginine is a basic amino acid. Thus the most protonated form of vasopressin has two positive charges. The first pKa thus corresponds to the first positive charge, and the second pKa is the second positive charge. pI is the point at which you have a neural molecule, which is roughly the average of the pKa that corresponds to the last positive charge, and the pKa that give a singly negative charged molecule. That might have been one of the more convoluted explanations yet.

I think if you're ever given a case with 3 dissociable hydrogens like this, to find the pI, average out the highest 2 pKa (in the case of a basic compound) or the lowest 2 pKa (in the case of an acidic compound)

Example:

Aspartic Acid:
pKa of carboxylic group = 2.10
pKa of amino group = 9.82
pKa of side chain = 3.86

pI = (2.1 + 3.86)/2 = 2.98

Lysine:
pKa of carboxylic group = 2.18
pKa of amino group = 8.95
pKa of side chain = 10.53

pI = (8.95 + 10.53)/2 = 9.74

 

[sangria1986]

The pI is about 11.3. Arginine is a basic amino acid. Thus the most protonated form of vasopressin has two positive charges. The first pKa thus corresponds to the first positive charge, and the second pKa is the second positive charge. pI is the point at which you have a neural molecule, which is roughly the average of the pKa that corresponds to the last positive charge, and the pKa that give a singly negative charged molecule. That might have been one of the more convoluted explanations yet.

I don't follow...why is it not the first two since it's acidic?

 

[Rabolisk]

Because the most protonated form (pH << 8) has +2 charge. You know this because arginine and the N-terminus are both NH3+, whereas the hydroxyl is just OH. At pKa1, you have equal concentration of +2 and +1. At pKa2 you have an equal concentration of +1 and neutral, and at pKa3, you have an equal concentration of neutral and -1.

 

[nUgnoY]

I just want to point out that guanidino group is NOT the N terminus of the arginine. And NT would be in a peptide bond in vasopressin, so it wouldn't be basic to begin with (it wouldn't do acid-base). Guanidino is the di-amine-imino group on the R chain or arginine.

Here's a process that's very simple to me:

1. Protonate everything.
Charge? +2*
2. Deprotonate it once. lowest pKa H will leave (N-term).
Charge = +1
3. Deprotonate it again. lowest pKa H will leave (Hydroxyl).
Charge = 0
4. Deprotonate it again. lowest pKa H will leave (guanidino)
Charge = -1

You want to find the average of the two sites that surround neutral charge. In this case, it's the hydroxyl and the guanidino. Average 10 and 12.5, 11.25.

*N-Term will be positive. OH will be neutral. Guanidino will be positive (you can deduce it since Arginine is + in physiological pH). Charge is +2

 

[zzzcor]

isn't pI 9...?
first pka1 = 2 or so for carboxylic acid chain so pka 2 will be 8 and next is 10

or am I just completely wrong?

 

[zzzcor]

oops never mind. it didn't have the carboxylic end from the beginning D: