Finding the charge of vasopressin (pI and pKa) TBR#4

[Zvousha]

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Hello!

I'm having trouble understanding how to find the charge of vasopressin at pH of 8.

The question provides the structure of vasopressin and states that the pKa of the N-terminus is 8, the phenyl hydroxide is 10, and the guanidino group 12.5. Then it asks for the net charge: +2, +1.5, +1, or +0.5

I understand how to find the pI (11.25) - but I'm not sure if that's useful here.

Really don't know how to approach the problem.
Any help/advice would be greatly appreciated! 🙂 Thank you!

 

[Teleologist]

Think about what happens when pKa = pH.

EDIT: it's probably easier to just use the pI.

 

[Zvousha]

How?

I thought about that too, but it didn't get me much further. According to Hendreson-Hasselbach, when pH=pKa, then the concentration of deprotonated molecules equals the concentration of protonated molecules, so the charge should be +0.5. But the answer is 1.5...?? 🙁

 

[Teleologist]

How?

I thought about that too, but it didn't get me much further. According to Hendreson-Hasselbach, when pH=pKa, then the concentration of deprotonated molecules equals the concentration of protonated molecules, so the charge should be +0.5. But the answer is 1.5...?? 🙁

Well, yeah, you have multiple groups with multiple pKa values.

Sorry I don't know how exactly to solve this either. I just know about this because I audited a biochem class 😛.

 

[Zvousha]

Well, yeah, you have multiple groups with multiple pKa values.

Sorry I don't know how exactly to solve this either. I just know about this because I audited a biochem class 😛.

Haha, thank you anyways 🙂 Hopefully someone knows!