28

chapter 2

Amino Acids

In basic solution (above pH 9.7), the predominant species

is negatively charged and migrates toward the anode:

H

O

I

II

R— C— C— 0~

I

n h

2

The

isoelectric point

(pi) of an amino acid is the pH

at which the molecule has an average net charge of

zero and therefore does not migrate in an electric field.

The pi is calculated by averaging the pK' values for the

two functional groups that react as the zwitterion becomes

alternately a monovalent cation or a monovalent anion.

At

physiological

pH,

monoaminomonocarboxylic

amino acids, e.g., glycine and alanine, exist as zwitterions.

F IG U R E

2-8

Titration profile of histidine.