How to find the net charge of this?

[mariposas905]

I think I'm missing basic details here, it's freaking me out.

How is the net charge of peptide C -1? When I put in the charges, I got +1, -1, -1, +1, +1, -1 which canceled to a zero net charge

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[pretty_positron]

I got this one wrong too and it's for a stupid reason that I suppose we have to know now.

I thought, like you, that since it's a basic AA that it should have a +1 charge.

The pKa for histidine is 6, so at pH 7 it should be deprotonated leaving it a -1 charge.

But apparently, the pI for a basic AA is [(pKa of amino group) + (pKa of R group)]/2 which ends up being something like 7.5.

When pH = pI, it's a zwitterion with a net charge of 0. So that's why.



AAMC is terrible at explaining anything.

 

[Brain Whisperer]

At normal pH conditions, only arginine(R) and lysine(K) are positive and only glutamate(E) and aspartate(D) are negative

 

[CommyO]

I got this one wrong too and it's for a stupid reason that I suppose we have to know now.

I thought, like you, that since it's a basic AA that it should have a +1 charge.

The pKa for histidine is 6, so at pH 7 it should be deprotonated leaving it a -1 charge.

But apparently, the pI for a basic AA is [(pKa of amino group) + (pKa of R group)]/2 which ends up being something like 7.5.

When pH = pI, it's a zwitterion with a net charge of 0. So that's why.



AAMC is terrible at explaining anything.

Your logic is generally correct when considering pKa vs pH, however His is +1 at physiological pH (pH of 7)


Isoelectric point (pI) is where it is a zwitterion, therefore; as you said, the pI is the (pKa of N terminus (amino) + pKR )/ 2

His, Lys, Arg are +2 at low pH (pH<~2)
This is because their second or third deprotonation is the R group

 

[pretty_positron]

Your logic is generally correct when considering pKa vs pH, however His is +1 at physiological pH (pH of 7)


Isoelectric point (pI) is where it is a zwitterion, therefore; as you said, the pI is the (pKa of N terminus (amino) + pKR )/ 2

His, Lys, Arg are +2 at low pH (pH<~2)
This is because their first deprotonation is the R group

Wait then why isn't that the answer then? I would think it was +1 too, but then the answer wouldn't make sense.

 

[CommyO]

Wait then why isn't that the answer then? I would think it was +1 too, but then the answer wouldn't make sense.

I don't know the full question of OP but His would not be -1 at pH of 7. It would be about +.5