section 2.3
Electrolyte and Acid-Base Properties
27
2.3 Electrolyte and
Acid-Base Properties
Amino acids are ampholytes, i.e., they contain both acidic
and basic groups. Free amino acids can
never
occur as
neutral nonionic molecules:
Nonpolar
neutral
-
R-group
Ammonium cation
H
R—c —c:
©
I
\
NH3
O
Zwitterion
Resonance-
«- stabilized
carboxylate
anion
H
o
II
R— C— C— OH
I
n h
2
Instead, they exist as neutral
zwitterions
that contain both
positively and negatively charged groups:
Zwitterions are electrically neutral and so do not migrate
in an electric field. In acidic solution (below pH 2.0), the
predominant species of an amino acid is positively charged
and migrates toward the cathode:
H
O
I
II
R— C— C— OH
I
NH3+
F IG U R E
2-7
Titration profile of glycine, a monoaminomonocarboxylic acid.
