Justify: An amino acid can act as buffer.

1. In common chemical usage, a pH buffer is a substance or a mixture of substances, that permits solutions to resist large changes in the pH upon addition of small amounts of H+ and OH- ions.

2. A common buffer mixture contains two substances, a conjugate acid (proton donor) & conjugate base (proton acceptor). Together two species conjugate acid and conjugate base resists large changes in pH by partially absorbing additions of H+ or OH- ions to the system.

3. Buffering results from two reversible reaction equilibria occurring in a solution of nearly equal concentration of proton donor and its conjugate proton acceptor.

4. All the amino acids possess a single α-amino group and a single α-carboxyl group and R-group.

5. So in amino acids, α-carboxyl group acts as conjugate acid (proton donor) and α-amino group acts as proton acceptor. Together α-carboxyl group and α-amino group resists change in pH by partially absorbing additions of H+ or OH- ions to the system.

6. Example : The titration curve of glycine has relatively flat zone extending about one pH unit on the either side of its midpoint, PI of 5.97. PI is the isoelectric pH of amino acid at which amino acid is in zwitter ion form in which they are neutral or amphoteric.

Figure 1: Titration curve of amino acid glycine showing shaded boxes centered about pk1= 2.34 and pk2 = 9.60 indicate the regions of greatest buffering power. pI of glycine is 5.97.

7. Titration curve of glycine indicates that amino acid has two regions of buffering power. So there are two stages in titration of glycine.

8. At the midpoint in the 1st stage of titration of glycine, equimolar concentrations of proton donor (+H3N-CH2-COOH) and proton acceptor (+H3N-CH2-COO-) species are present.

9. In this zone, (shaded area in graph) an amount of H+ or OH- ions added to the system has much less effect on pH that the same amount added outside the buffer range.

10. At midpoint of any titration, the pH is equal to pka of the protonated group being titrated.

11. At the midpoint in the 1st stage of titration of glycine, the pH is 2.34 thus its –COOH group has pka (labeled pk1 in figure 1) of 2.34.

12. At the midpoint of second stage of titration of glycine, the pH is 9.60 which is equal to pka (labeled pk2 in figure 1) for –NH3 group.

13. Titration of glycine completes at pH 12 at which predominant form of glycine is H2N-CH3-COO-

14. One of the buffering zone is relatively flat portion of the curve extending for approx. one pH unit on either side of the first pka of 2.34 indicating that glycine is good buffer near this pH.

15. The other buffering zone is centered around pH 9.60 (Note that glycine is not a good buffer at the pH of intracellular fluid or blood i.e 7.4)

16. Many other amino acid acts as buffer at different pH depending on their isoelectric pH and pka values.

17. Another example: Histidine with pka=6.0 acts as a buffer near neutral pH usually found in extracellular fluids of animals and bacteria.