642
Bioinorganic Chemistry
CH
3
Hg
+
, all the metal ions carry two
positive charges. Owing to the strongly
chelating bidentate ligands, glycine (gly)
and
1,2-diaminoethane
(en),
CH
3
Hg
+
with only a single strong binding site is
at a competitive disadvantage in the above
series. To a unidentate ligand, CH
3
Hg
+
binds more strongly than all of the above
metal ions except Hg
2
+
.
In the above series, each inequality
sign stands for an approximate 10-fold
increase in stability constant. The two
series
gly
and
en
are
similar,
their
major difference being a stability constant
span from Mg
2
+
to Hg
2
+
of 10
9
for
glycine and 10
14
for en. Generally, the
increment between metal ions increases
on passing from O
<
N
<
S donor atoms.
Thepresenceo
fasu
l
furdonorpromo
tes
Cd
2
+
and Pb
2
+
to higher positions than
that
in
the
above
series.
The
order
of
increasing
sulfur-binding
strengths
is
Zn
2
+
<
Cd
2
+
<
Pb
2
+
<
CH
3
Hg
+
<
Hg
2
+
. Sulfhydryl group interactions are
t
h
em
a
i
nm
o
d
eo
ft
o
x
i
c
i
t
yo
ft
h
e
heavy-metal ions. These general bonding
features are expressed in the concept of
the stability ruler.
6
Stability Ruler
For a single ligand, the Irving–Williams
stability sequence of dipositive metal ions
is Mg
<
Mn
<
Fe
<
Co
<
Ni
<
Cu
>
Zn,
invariant of ligand. The uniformly pro-
gressive part of this sequence from Mg
2
+
to Cu
2
+
de±nes a
stability ruler
to which
more variable metal ions may be com-
pared. The stability ruler appears across
the top of Table 4. For several ligand donor
sets, Table 4 shows the relative binding
strengths of the variable dipositive metal
ions Zn
2
+
,Cd
2
+
,Pb
2
+
,andHg
2
+
.The
ir
placement in Table 4 corresponds to their
stability constants compared with those of
the metal ions that de±ne the ruler. The
entries for glycine and en agree with the
series in the previous section. With most of
the metal ions in Table 4, histidine will be
tridentate. The increment between metal
ions and hence the length of the stabil-
ity ruler increases with the substitutions
O
<
N
<
S. The ruler length appears in
the second to last column of Table 4 in
log
K
units for binding to each ligand. The
longer the ruler, the more discriminating
Tab. 4
Stability ruler.
a
Ligand
Donors
Mg
<
Mn
<
Fe
<
Co
<
Ni
<
Cu
Length
b
Hg
c
Hydroxide
OH
Cd
Zn
Pb
3.7
1.2
Acetate
O
Zn
Cd
Pb
1.3
1.2
Imidazole
=
NZ
n
C
d
4
.
0
1
.
1
Ammonia
NH
3
Pb
Zn
Cd
4.0
1.1
Oxalate
O
,O
Cd
Zn
Pb
2.1
Glycine
N,O
Cd
Pb
Zn
6.1
0.4
Histidine
N,
=
NC
d
P
b
Z
n
8
.
2
0
.
2
NH
2
(CH)
2
NH
2
N,N
Pb
Cd
Zn
10.1
0.4
NH
2
(CH)
2
S
N,S
Zn
Cd
Pb
12(est.)
0.2
a
All dipositive cations.
b
Ruler length in difference between log stability constants of Cu
2
+
and Mg
2
+
complexes.
c
Relative magnitude that the Mg
2
+
to Cu
2
+
length must be extended to reach the Hg
2
+
value.
previous page 642 Encyclopedia of Molecular Cell Biology and Molecular Medicine read online next page 644 Encyclopedia of Molecular Cell Biology and Molecular Medicine read online Home Toggle text on/off