Return to Contents Chapter 27: Amino Acids, Peptides, Proteins and Nucleic Acids Ch 27 contents

Structure and pKa of Amino Acids

QUESTION Can you think of a very simple, common compound that is amphoteric ? ANSWER
The equations that define acidity and basicity are: 
equation defining Bronsted acidity
equation that defines K<sub>a</sub>  
equation that defines pK<sub>a</sub>
Remember that the lower the pKa, the stronger the acid. ( review ?)

From these expressions it is possible to derive the important Henderson-Hasselbalch equation :

  pKa = pH + log [HA] / [A-]

How does this equation help us ?


These principles can be extended to poly acidic / basic systems (such as amino acids) by thinking of each pKa value in turn.
This information will let you decide which structure of an acid or base will dominate at a particular pH.

Let's look at an example.

To the left are the processes for the amino acid HISTIDINE which has an extra basic group.

It has three acidic groups of pKa's 1.82 (carboxylic acid), 6.04 (pyrrole NH) and 9.17 (ammonium NH).
Histidine can exist in the four forms shown, depending on the solution pH, from acidic pH (top) to basic pH. (bottom).

Starting from the top, we can imagine that as we add base, the most acidic proton is removed first (COOH), then the pyrrole NH then finally the amino NH. These takes us through each of the forms in turn.

At pH < 1.82, A is the dominant form.

In the range 1.82 < pH < 6.02 B is the dominant form.

In the range 6.02 < pH < 9.17 C is the dominant form, and

when pH > 9.17, D is the major form in solution.

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organic chemistry © Dr. Ian Hunt, Department of Chemistry University of Calgary