objectives

Buffering capacity of amino acids and proteins

Recall that amino acids and proteins have acid-base behaviour, due to the presence of amino and carboxyl functional groups. There are several R groups that also have acid-base activity, resulting in proteins being influenced by changes in the pH of their aqueous environment. When the pH of the environment changes, the interactions between amino acid residues that form the basis for protein folding can affect the overall shape of the protein, resulting in changes to the protein's function.

Buffer solutions

With proteins being sensitive to changes in pH, the aqueous environment where a protein performs its functions must be maintained. Often, a very narrow pH range is required or the protein will denature and lose both shape and function. Buffer systems are solutions that resist changes in pH when small amounts of acid or base are added. In a cell or organ where proteins are present, buffer systems naturally exist to minimise any changes to pH when acids or bases are produced during metabolic processes. When proteins are studied outside of the organism, buffer solutions must be made to maintain the pH required by the protein.

The presence of both amino (-NH2) and carboxyl (-COOH) functional groups on amino acids results in the amino acids being able to buffer a solution.

When polymerised as a protein, the amino and carboxyl functional groups from the amino acids no longer have acid-base behaviour. The acidic and basic functional groups on the R groups of some amino acid residues in proteins mean that the proteins themselves can buffer a solution.

Buffer solutions can also be made by using a weak acid and adding the salt of the conjugate base, such that the resulting concentrations are nearly equal, resulting in a solution that will be buffered at a pH below 7. Conversely, a weak base and the salt of the conjugate acid could also be used to make a buffer that has a pH above 7.

For example, the blood uses a system of hydrogen carbonate ions (HCO3-) in equilibrium with carbonic acid (H2CO3) to maintain the pH of blood at 7.4. The hydrogen carbonate ions are the conjugate base of the weak carbonic acid. The concentration of carbonic acid is directly influenced by the concentration of carbon dioxide levels in the blood, which is controlled by breathing.

If the pH of the blood decreases, due to a higher concentration of hydronium ions, the equilibrium will shift to the left, using up the excess acid. If the pH increases, due to a lower concentration of hydronium ions, then the equilibrium will shift to the right to replenish the acid.

Calculating the pH of a buffer

The pH of a buffer solution can be calculated using the Henderson-Hasselbalch equation.

The pKa is an expression of the strength of the weak acid, in this case H2CO3. The larger the value, the weaker the acid. Carbonic acid has a pKa of 6.35. [A-] is the concentration of the conjugate base, HCO3-, and [HA] is the concentration of the acid, H2CO3.

For example: A student is making a sample of simulated gastric juice in the stomach by preparing a buffer for an experiment. If 0.0200 dm3 of 0.10 mol dm-3 carbonic acid is added to 0.0100 dm3 of 0.10 mol dm-3 sodium hydrogen carbonate, what is the pH of the buffer solution?

Firstly, the new concentrations must be calculated for dilution in the new total volume:

The concentration of carbonic acid:

The concentration of sodium hydrogen carbonate:

Using the Henderson-Hasselbalch equation: