Chemistry Reference
In-Depth Information
17.29
+
+
NH
3
NH
3
p
K
1
= 2.1
9
CO
2
-
HO
2
CCH
2
CH
2
CH
CO
2
H
HO
2
CCH
2
CH
2
CH
p
K
2
= 4.25
+
NH
2
NH
3
p
K
3
= 9.6
7
CO
2
-
CO
2
-
-
-
O
2
CCH
2
CH
2
CH
O
2
CCH
2
CH
2
CH
17.30
NH
2
NH
2
-
p
K
3
= 12.
48
-
H
2
N
C
NH(CH
2
)
3
CH
CO
2
H
2
N
C
NH(CH
2
)
3
CH
CO
2
NH
NH
2
+
p
K
2
= 9.04
+
+
NH
3
NH
3
p
K
1
= 2.1
7
-
H
2
N
C
NH(CH
2
)
3
CH
CO
2
H
H
2
N
C
NH(CH
2
)
3
CH
CO
2
NH
2
NH
2
+
+
The order of basicities (and therefore the sequence of protonation of groups) is
guanidine > -NH
2
> -CO
2
-
.
17.31
In strong acid (pH 1), histidine has two resonance contributors:
H
H
+
H
H
N
CH
2
C
CO
2
H
CH
2
C
CO
2
H
+
N
+
H
3
N
+
H
3
H
H
17.32
When an amino acid is in aqueous solution at a pH greater than its isoelectric point
(pI), it will be negatively charged and will migrate toward the anode (positive
electrode) of an electrophoresis apparatus. If the pH is less than the pI, the amino
acid will migrate toward the cathode (negative electrode). The p
I
of Gln, His, and Glu
are 5.7, 7.6, and 3.2, respectively (consult Table 17.1). Therefore, at pH 6, His will
migrate toward the cathode, while Gln and Glu will migrate toward the anode. Glu will
migrate faster than Gln because it will be more negatively charged at pH 6.
