Why does the isoelectric point decrease with acidity of amino acid?

[SamuelTesla]

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In an amino acid titration, the more acidic the side group, the lower the isoelectric point (pI) - can someone please explain in simple terms why this is?

 

[NickNaylor]

If something is more acidic, then its pKa is smaller and it has a smaller pH. Since the pI is essentially the average of the pH's of the amino acid in the +1 and the -1 state, a lower pH for the -1 state will decrease the average (a.k.a., the pI).

 

[fenguin]

In an amino acid titration, the more acidic the side group, the lower the isoelectric point (pI) - can someone please explain in simple terms why this is?

The isoelectric point is when there is one positive and one negative charge on the amino acid. The charges are on the amine group and carboxylic acid group that is not on the side chain.

At this point, the side chain carboxylic acid group is still uncharged (-COOH). It is therefore in its acid form and will decrease the pH and the pI. A stronger acid on the side group will decrease pI further.

 

[SN2ed]

Thread moved. Questions of this type belong in the Study Q&A forum.