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708 Chapter 21
Copyright © 2017 Pearson Education, Inc.
9.
a.
The carboxyl group of the aspartic acid side chain is a stronger acid than the carboxyl group
of the glutamic acid side chain. The carboxyl group of the aspartic acid side chain is closer to the
electron-withdrawing protonated amino group.
b.
The protonated lysine side chain is a stronger acid than the protonated arginine side chain. The
protonated arginine side chain has less of a tendency to lose a proton because its positive charge is
delocalized over three nitrogens.
10.
11.
a.
aspartate
1
pI
=
2.98
2
b.
arginine
1
pI
=
10.76
2
c.
Aspartate because its pI is lower than that of glutamate. Therefore, pH 6.20 is farther away from the
pH at which it has no net charge. The farther an amino acid with no net charge has to “move” to get to
the given pH value, the more charged it will become.
d.
Methionine because at pH
=
6.20, methionine is farther away from the pH at which it has no net
charge
1
pI of methine
=
5.75, pI of glycine
=
5.97
2
.
12.
Tyrosine and cysteine each have two groups that are neutral in their acidic forms and negatively charged in
their basic forms. Unlike other amino acids that have similarly ionizing groups, the p
K
a
value of one of the
two similarly ionizing groups in tyrosine and in cysteine is close to the p
K
a
value of the group that ionizes
differently. Therefore, the group that ionizes differently cannot be ignored in calculating the pI.
13.
a.
< 25%
b.
> 75%
14.
For the amino acid to have no net charge, the two amino groups must have a
+
1 charge between them in order to
cancel out the
-
1 charge of the carboxylate group. Because they are positively charged in their acidic forms and
neutral in their basic forms, the sum of their charges will be
+
1 at the midpoint of their p
K
a
values.
15.
CH
3
CH H
The R group of the aldehyde is the same
as the R group of the amino acid.
CH
3
C
O
16.
Leucine and isoleucine both have C
4
H
9
side chains and, therefore, have the same polarity. Consequently,
the spots for both amino acids appear at the same place on the chromatographic plate. Therefore, the
chromatographic plate has one less spot than the number of amino acids.
17.
Because the amino acid analyzer contains a cation-exchange resin (it binds cations), the less positively
charged the amino acid is, the less tightly it is bound to the column. Using buffer solutions of increasingly
higher pH to elute the column causes the amino acids bound to the column to become increasingly less
positively charged, so they can be released from the column.
a.
asparagine pI
=
2.02
+
8.84
2
=
10.86
2
=
5.43
b.
arginine pI
=
9.04
+
12.48
2
=
21.52
2
=
10.76
c.
serine pI
=
2.21
+
9.15
2
=
11.36
2
=
5.68
d.
aspartate pI
=
2.09
+
3.86
2
=
5.95
2
=
2.98