logo.gif (3296 bytes)Introduction to Protein Structure

Acid / Base Chemistry of Amino Acids


Both the amino group and the carboxyl group of each amino acid are ionizable. The carboxyl group (with a pKa of about 3) is deprotonated at physiological pH. The amino group is protonated at physiological pH. (The pKa of ammonium ions is about 9.)

Thus, amino acids exist at zwitterions ("twin ions") at physiological pH:

wpe20.jpg (6194 bytes)

In addition, the side chains of some of the amino acids contain ionizable groups.

Amino Acid pKa

Functional Group

Acid/Base Chemistry

Aspartate & Glutamate 4.4 carboxyl

wpeA.jpg (2431 bytes)

Histidine 6.5 imidazole wpeF.jpg (3211 bytes)
Cysteine 8.5 sulfhydryl wpeB.jpg (1742 bytes)
Lysine 10.0 amine wpeC.jpg (1943 bytes)
Tyrosine 10.0 phenol wpeD.jpg (3360 bytes)
Arginine 12.0 guanidinium wpeE.jpg (3490 bytes)

Examine the titration behavior of the amino acids in detail by going to this site at the University of Virginia.


Copyright 1998, 1999, 2007 by Frank R. Gorga;   Page maintained by F.R. Gorga;   Last updated: 12-Mar-2007