Again- which group ionizes first?
It still depends on the chemical nature of the group and the general rules apply
(acids ionize first, amino groups second).
But some amino acids have two groups of similar chemical nature and
some additional information about the whole molecule must be used to know which ionizes first.
Use the rules below to help you figure out which group ionizes 1st, 2nd, 3rd etc. in complex amino acids.
Additional Rules:
3. Carboxylic groups near an amino group in a molecule have more acidic pK than isolated carboxylic acid groups.
Therefore, alpha carboxyl groups ionize before side chain carboxylic acids.
4. Amino groups near a carboxylic acid group also have more acidic pK than isolated amines.
Therefore, alpha amino groups ionize before side chain amino groups.
5. Aromatic amines like in His have pK about pH 6.
©Wilbur H. Campbell, 1995, 1996; wcampbel@mtu.edu
Back To: Lecture 3 Acids, Bases and Buffering