BL/CH 401 Lecture #3 - Acids, Bases, and Buffers

5. Ionization of Complex Amino Acids:

Most of the 20 amino acids are like Gly in their ionization properties

because their side chains do not ionize at biological pHs.

However, 5 amino acids have side chains that ionize (really 2 others do also but we ignore them).

The 5 Complex Amino Acids are:

glutamic acid (Glu), aspartic acid (Asp), lysine (Lys), arginine (Arg) and histidine (His).

Each of these 5 amino acids has 3 ionizable groups and therefore, 3 pKs.

The pKs for these 5 amino acids are shown below in the order they ionize:

Glu - pKs = 2.1 (alpha-carboxylic acid), 4.1 (side chain-carboxylic acid), 9.5 (amino)

Asp - pKs = 2.0 (alpha-carboxylic acid), 3.9 (side chain-carboxylic acid), 9.9 (amino)

Lys - pKs = 2.2 (alpha-carboxylic acid), 9.2 (amino), 10.8 (side chain-amino group)

Arg - pKs = 1.8 (alpha-carboxylic acid), 9.0 (amino), 12.5 (side chain-amino group)

His - pKs = 1.8 (alpha-carboxylic acid), 6.0 (side chain-aromatic amino group), 9.3 (amino)

All 5 amino acids with ionizable groups shown together like in the class handout:

Go To: Help for Ionization of Amino Acids

©Wilbur H. Campbell, 1995, 1996; wcampbel@mtu.edu

Back To: Lecture 3 Acids, Bases and Buffering