Protonation of amino acids – How does the Pka (=pks) value change?

kaktusfishBykaktusfish

Assumption: An amino acid with a COO group and an aromatic amino acid such as phenylalanine meet inside a protein, for example. As a result, COO is protonated to COOH.

My question now is: Why is the pKa value increasing? I can't really explain it. When pKa=PH, half of a molecule is protonated and the other half is deprotonated. Furthermore, a low pKa value means a strong acid is present. In this case, with the formation of COOH, shouldn't the pKa value decrease rather than increase, since the overall acidity is becoming more acidic?

I would be very happy if someone could help me out here :)!!

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anwesende
anwesende
1 year ago

I think you're confusing pK's value and pH.

At an AS you have an amino group that can be protonated and a carbonic acid group that can be deprotonated. This can easily be achieved by adding a strong acid or base. But only the pH value of the solution changes, not the pK value of AS.

If instead of giving a second AS to the first, the stronger acid protonates the other. The pK's value tells you who protons, but he doesn't change.

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