650
Bioinorganic Chemistry
Tab. 5
Conventional and conditional (pH 7.0) stability constants.
Ligand
p
K
a
Ni
2
+
Zn
2
+
Cd
2
+
log
K
s
log
K
7
log
K
s
log
K
7
log
K
s
log
K
7
Acetate (O
)
4.7
0.7
0.7
0.9
0.9
1.3
1.3
Imidazole (
=
N)
7.11
3.0
2.6
2.52
2.2
2.80
2.4
Glutathione (RS
)
8.92
4.0
2.1
5.0
3.1
6.16
4.2
Ammonia (NH
3
)
9.33
2.79
0.5
2.31
0.0
2.65
0.3
Glycine
9.68
5.80
3.1
5.03
2.3
4.28
1.6
Histidine
9.15
8.7
6.5
6.6
4.4
5.4
3.2
6.10
1,2-Diaminoethane
10.0
7.3
3.9
5.7
2.3
5.4
2.0
7.2
Glutathione is the tripeptide
γ
-L-glutamyl-
L-cysteinylglycine, and it serves as a superb
model for unhindered sulfhydryl groups
in proteins.
The results in Table 5 show that for all
three metal ions, the ligand stabilities from
the conventional constants follow the in-
creasing sequence acetate
<
ammonia
<
imidazole
<
glutathione. This sequence
differs from the stability order in neu-
tral solutions because the sequence applies
only when all ligands are predominantly in
their basic form, which is true only for ac-
etate. For conditional constants at pH 7.0,
Table 5 shows that for Ni
2
+
the increas-
ing stability order is ammonia
<
acetate
<
glutathione
<
imidazole, while for Zn
2
+
and
Cd
2
+
the
order
is
ammonia
<
acetate
<
imidazole
<
glutathione. For all
three metal ions in neutral solutions, the
most basic ammonia has become a weaker
ligand than the least basic acetate. The
stability order from conditional constants
suggests that in neutral solutions, it is
apt for Ni
2
+
to be found at imidazole
sites and Zn
2
+
and Cd
2
+
at sulfhydryl
groups. Of course, chelation by adjacent
donors may increase stabilities and alter
preferred binding sites. Yet, the exam-
ples in Table 5 serve as models for the
analogous groups in proteins and prove
that binding strengths and stability orders
are pH-dependent.
In the nucleosides of the 6-oxopurines,
guanosine and inosine, in neutral solu-
tions, the much more basic N1 site is
protonated while the N7 site is dif±cult to
protonate. Metal ions coordinate in acid
solutions at N7, and as the pH increases,
they compete better with the proton and
crossover to N1.
The
principles
developed
in
this
section extend to ligands with several
basic sites. For diamines, EDTA, and
catecholates in which metal ions displace
two protons in neutral solutions, the
fraction of unbound, doubly deprotonated
ligand
becomes
α
=
K
a1
K
a2
/[(H
+
)
2
+
(
H
+
)
K
a1
+
K
a1
K
a2
], where p
K
a1
<
p
K
a2
.
Table 5 also concludes with two bidentate
amine ligands and histidine. For 1,2-
diaminoethane, the conditional stability
constants at pH 7.0 are 3.4 log units less
than tabulated stability constants.
Ignored in this section and Table 5
is the formation of metal ion hydroxo
complexes and precipitates. Formulation
of
conditional
stability
constants
that
incorporate hydroxo complex formation is
outside the scope of this section.
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