Chemistry Reference
In-Depth Information
These internal salts are known by the German word
zwitterion
(German for 'dipolar ion'), and formation of the zwitterion makes the
amino acid very polar and therefore very soluble in water. If acid is added
to the zwitterion, the ionised COO
group will accept a proton to give
undissociated COOH. The overall charge on the amino acid will now be
positive, due to the NH
3
. Similarly, if base is added to the zwitterion, the
NH
3
(which is really the conjugate acid of NH
2
) will function as an acid
and donate its proton to the base. The overall charge on the amino acid will
now be negative, due to the ionised COO
. Amino acids are, therefore,
ionised at all values of pH. They are positively charged at low pH, nega-
tively charged at high pH and zwitterionic at neutral pH. The fact that
amino acids are ionised at all values of pH and are zwitterionic at neutral
pH has profound implications for the oral absorption and bioavailability of
amino acids from the diet. The body has to resort to specialised uptake
mechanisms to ensure that sufficient levels of these essential nutrients are
absorbed (see Chapter 2). The ionisation of
the simplest amino acid,
glycine, is represented in Figure 1.3.
COO
-
H
COO
-
H
COOH
H
-H
+
-H
+
C
C
C
+
+
H
H
H
N
H
3
N
H
3
NH
2
Figure 1.3
The ionisation of glycine.
If the pH of the protein or amino acid solution is adjusted so that the
number of ionised COO
groups is equal to the number of ionised NH
3
groups, then that value of pH equals pI, the
isoelectric point
of the protein
or amino acid. This point corresponds to the minimum solubility of the
protein, and the point at which migration of the protein in an electric field
is slowest (as in the technique of
electrophoresis
, which is used to separate
mixtures of proteins according to their overall electrical charge). The
isoelectric point for an amino acid may be easily calculated if the p
K
a
values
for the NH
3
and COO
are known (e.g. by titration). For a simple amino
acid, such as glycine, the pI is simply the average of the two p
K
a
values. For
more complex amino acids, such as glutamic acid or arginine which have
ionisable groups in the side-chains, the pI is given by averaging the two p
K
a
values that lie on either side of the zwitterion. This is true no matter how
many times an amino acid or peptide ionises. For an amino acid with one
acidic group on the side-chain, there are three distinct ionisations and hence
