Chemistry Reference
In-Depth Information
three distinct p
K
a
values. Fully protonated aspartic acid ionises as shown in
Figure 1.4.
COO
-
H
+
H
3
COOH
H
+
H
3
p
K
a1
=1.88
+ H
+
HOOC
C
HOOC
C
COO
-
H
COO
-
H
p
K
a2
= 3.65
+ H
+
HOOC
C
- OOC
C
+
H
3
+
H
3
COO
-
H
COO
-
H
p
K
a3
=9.60
+ H
+
- OOC
C
- OOC
C
+
H
3
NH
2
COO
-
Na
+
H
C
HOOC
NH
2
MSG
Figure 1.4
The ionisation of aspartic acid and structure of MSG.
The first group to ionise (and hence the strongest acid) is the COOH
group on the a carbon. This gives p
K
a1
. The second proton is lost from the
side-chain COOH to give p
K
a2
. Finally, the NH
3
on the a carbon ionises to
give p
K
a3
. There is, of course, only one pI, which is given by the average of
the two p
K
a
values on either side of the zwitterion, i.e.
4
p
K
a2
).
The other commonly occurring amino acid with an acidic side-chain
is glutamic acid. This compound is probably best known as its monosodium
salt (monosodium glutamate or MSG). This salt is added to foods (espe-
cially oriental food) to enhance the flavour and impart a 'meat-like' taste to
the food. Interestingly, both the
D
enantiomer of glutamic acid and the
naturally occurring
L
form are used as food additives. Use of the non-
natural
D
isomer may account for some of the adverse reactions experienced
by consumers of MSG in food.
(p
K
a1
