Amino acids buffering capacity

[doctorwannabee]

Currently studying for the MCAT and having some confusion about the buffering capacity of amino acids. So, I understand that at pI the amino acid has no overall charge, but I don't understand why at pI an amino acid has minimal buffering capacity. It should still be protonated at the carboxylic end, which would be deprotonated at pI, if you add an acid or deprotonated at the amino end, which would be fully protonated at pI, if you add a base. So why does it have minimal buffering capacity at pI? Am I missing something? I would really appreciate it if someone could explain the reason behind it.
Thanks a lot!

---

Members don't see this ad.

 

[Czarcasm]

Currently studying for the MCAT and having some confusion about the buffering capacity of amino acids. So, I understand that at pI the amino acid has no overall charge, but I don't understand why at pI an amino acid has minimal buffering capacity. It should still be protonated at the carboxylic end, which would be deprotonated at pI, if you add an acid or deprotonated at the amino end, which would be fully protonated at pI, if you add a base. So why does it have minimal buffering capacity at pI? Am I missing something? I would really appreciate it if someone could explain the reason behind it.
Thanks a lot!

Are you sure you weren't given a reaction condition where say, for example, a reaction occurs at a pH of 5 and your buffer (amino acid has a pI of say 7). In this instance, this would not be a suitable buffer only because the buffering capacity for the amino acid (or any buffer for that matter) is within a +1/-1 pH range. Outside of that it longer functions as a buffer. It shouldn't have anything to do with the pI. All pI is is the point at which the amino acid exists in its zwitterionic, neutral charged form. However, this species has a pKa of it's own which is basically just an average of the two close valued pKa groups.

In other words, to be an ideal buffer, you would want your reaction to have a pH equivalent to the pI of your amino acid. In this scenario, the amino acid would function most optimally as a buffer for the proceeding reaction.

 

[doctorwannabee]

thank you! that's what I was thinking with regards to the reaction having a pH equivalent to the pI of the amino acid. I got confused because I am currently enrolled on the Kaplan MCAT course and one of their flashcards says that amino acids have minimal buffering capacity at their pI which wasn't making much sense to me.

 

[Paean1]

At the isoelectric point the amino acid's amine group is protonated and its carboxylic group is deprotonated.

The amine group has a pKa around 9. The carboxylic group has a pKa around 2.

If we simplify this by ignoring side chains, an amino acid will act as a buffer in the pH region around either of those two pKa values. The buffer is essentially between NH3/NH2 around a pH of 9, and between COOH/COO- around a pH of 2.

Lets assume the pI for an amino acid is around 5.5.

If a solution of amino acids at the pI has acid added to it, the pH will decrease until it has reached the pH region around the pKa value of carboxylic acid.

Similarly, if a solution of amino acids at the pI has base added to it, the pH will increase until it has reached the pH region around the pKa value of the amine.

In both cases the solution's pH changes drastically before the amino acids begin to act as a buffer. So, amino acids at their pI would be considered to have a low buffer capacity.

 

[doctorwannabee]

At the isoelectric point the amino acid's amine group is protonated and its carboxylic group is deprotonated.

The amine group has a pKa around 9. The carboxylic group has a pKa around 2.

If we simplify this by ignoring side chains, an amino acid will act as a buffer in the pH region around either of those two pKa values. The buffer is essentially between NH3/NH2 around a pH of 9, and between COOH/COO- around a pH of 2.

Lets assume the pI for an amino acid is around 5.5.

If a solution of amino acids at the pI has acid added to it, the pH will decrease until it has reached the pH region around the pKa value of carboxylic acid.

Similarly, if a solution of amino acids at the pI has base added to it, the pH will increase until it has reached the pH region around the pKa value of the amine.

In both cases the solution's pH changes drastically before the amino acids begin to act as a buffer. So, amino acids would be considered to have a low buffer capacity.

That makes perfect sense! Thanks so much!