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 pK_a of about 3) is deprotonated at physiological pH. The amino group is protonated at physiological pH. (The pK_a of ammonium ions is about 9.)

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

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
Histidine	6.5	imidazole
Cysteine	8.5	sulfhydryl
Lysine	10.0	amine
Tyrosine	10.0	phenol
Arginine	12.0	guanidinium

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