Chemistry Reference
In-Depth Information
10
Cu
2+
9
8
7
6
Zn
2+
Co
2+
5
4
Sr
2+
Ca
2+
Cd
2+
3
Mn
2+
2
1
0
1
1.5
2
2.5
3
2
r
FIGURE 1.5
Elimination of metal ions from crayfish hemolymph is related to metal ion
softness. Strong covalent bonding slows elimination. In this illustration, the iron datum was
omitted because it was derived from the citrate salt, whereas the other metals were pre-
pared from chloride salts. Based on trends shown here and in Figure 1.3, one could incor-
rectly assume that metal softness might always be the best descriptor for predicting trends.
However, as Ahrland explains, “soft and polarizable are not synonymous; a soft acceptor is
certainly always polarizable, but a highly polarizable acceptor need not necessarily be soft,
i.e., have (b) properties. For metal ion acceptors, the outer d-electrons are as essential as the
polarizability” (Ahrland, S. 1968. Thermodynamics of complex formation between hard and
soft acceptors and donors.
Struct. Bond
. 5:118 -149).
as the covalent nature of the metal bond with biochemical ligands increased: strong
covalent bonding slowed elimination.
1.3.2 B
iomolecule
-
to
-o
rgAnism
m
AnifestAtions
of
m
etAl
t
oxicity
It should be no surprise to the reader at this point that metal binding differences have
also been used to explain intermetal differences in toxicity. The simplest of such
effects, in vitro inhibition of enzyme catalysis, can be related to metal affinity to
intermediate ligands such as those with oxygen donor atoms (Newman et al. 1998)
(
Figure 1.6
). Examining several enzyme inhibition data sets, Newman et al. (1998)
suggested that they could best be modeled with the absolute value of the logarithm of
the first hydrolysis constant (i.e., K
OH
for M
n+
+ H
2
O → MOH
n−1
+ H
+
), although clear
trends also emerge if plotted against the softness index (σ
p
). The |Log K
OH
| reflected
metal ion binding affinity for intermediate ligands.
Expanding outward on the biological hierarchy scale from biomolecules to
cells, additional mechanisms emerge that could produce differences in metal ion
toxicity. Mechanisms include intermetal differences in transport, disruption of
ion regulation, binding to and altering protein or nucleic acid functioning, and
oxyradical generation. Newman et al. (1998) found coordination chemistry-based
trends in metal lethality to cultured cells from fish (Babich et al. 1986; Babich and
Borenfreund 1991; Magwood and George 1996) and hamster (Hsie et al. 1984),
bacterial bioluminescence suppression (McCloskey et al. 1996; Newman and
