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Chapter 21 709
Copyright © 2017 Pearson Education, Inc.
18.
Cation-exchange chromatography releases amino acids in order of their pI values. The amino acid with the
lowest pI is released first because at a given pH, it is the amino acid with the highest concentration of nega-
tive charge, and negatively charged molecules are not bound by the negatively charged resin. The relatively
nonpolar resin releases polar amino acids before nonpolar amino acids.
a.
Asp
1
pI
=
2.98
2
is more negative at pH
=
4 than is Ser
1
pI
=
5.68
2
.
b.
Ser is more polar than Ala.
c.
Val is more polar than Leu.
d.
Tyr is more polar than Phe.
19.
A column containing an anion-exchange resin releases amino acids in reverse order of their pI values (the
opposite of what happens in a cation-exchange resin). The amino acid with the highest pI is released first
because, at a given pH, it will be the amino acid with the highest concentration of positive charge.
His
7
Val
7
Ser
7
Asp
20.
The first equivalent of ammonia will react with the acidic proton of the carboxylic acid to form an ammo-
nium ion, which is not nucleophilic and, therefore, cannot substitute for Br. Thus, a second equivalent of
ammonia is needed for the desired nucleophilic substitution reaction.
21.
a.
The following reactions show that pyruvic acid forms alanine, oxaloacetic acid forms aspartate, and
a
-ketoglutarate forms glutamate.
O
O
O
_
pyruvic acid
enzyme
O
O
_
alanine
NH
3
+
O
O O
O
_
O
_
oxaloacetic acid
O
O
O
O NH
3
_
_
aspartate
+
enzyme
enzyme
O
O
O
O
_
O
_
-ketoglutaric acid
O
O
O
O
_
_
glutamate
NH
3
+
If reductive amination (page 764) is carried out in the cell, only the l-isomer of each amino acid will
be formed.