Bioinorganic Chemistry
639
to differences in concentration from a
surrounding milieu.
Essentiality is not the only criterion
for inclusion of elements in a survey
of
bioinorganic
chemistry.
Organisms
may accumulate elements without their
being essential. Some elements such as
arsenic and antimony have been used
therapeutically, and bismuth still is. Others
such as Al and the heavy metals Cd, Hg,
Tl, and Pb are toxic and prevalent, and
their interactions with life processes are
of concern.
2
Abundances
In the natural environment, most essential
elements
are
relatively
abundant
and
most toxic elements are relatively rare.
Abundant crustal elements that have not
become essential, such as Si, Al, and Ti,
are not easily accessible. Life evidently
evolved by using elements that are not only
abundant but also accessible and suitable.
Some relatively common elements are
not used in living organisms because
similar roles are played by more common
elements; Cl is 10
3
times more abundant
than Br, Ca is 100 times more abundant
than Sr, and Na and Mg are both 100 times
more abundant than Li. Boron, the 11th-
ranking element in oceans, is essential for
some plants but not animals. Essential Mo,
I, and Se do not even occur in the upper
50th percentile in crustal atom abundance,
but Mo ranks 20th in abundance in the
oceans and is the most abundant transition
metal ion in seawater.
Of the most toxic elements – arsenic
(As), cadmium (Cd), mercury (Hg), thal-
lium (Tl), and lead (Pb) – only the last
occurs in the upper 50th percentile in the
crust, and As and Cd occur similarly in the
oceans. Thus, the most toxic elements are
much rarer on the whole than the essen-
tial elements. However, both in the crust
and in the oceans essential Mo and toxic
As appear in comparable amounts as do
essential Se and toxic Cd.
3
Ionic Radii
Like size usually plays a greater role than
identical charge in promoting metal ion
substitutions in biology. Table 3 presents
effective
ionic
radii
for
many
metal
ions.
For
any
metal
ion,
the
radius
increases with coordination number, since
the greater number of bonds weakens
the
strength
of
any
one
bond.
The
radius of the most common coordination
number is underlined in Table 3. The
alkali and alkaline earth metal ions exhibit
variable coordination numbers without
strong directionality in bonding. Because
they are of similar size, Ca
2
+
and Na
+
of differing charges often interchange
in biological systems. Ba
2
+
is poisonous
because of its antagonism to K
+
of similar
radius and not to Ca
2
+
of identical charge
but smaller radius. Ba
2
+
is a muscle poison
for which one antidote is intravenous
infusion of K
+
.
4
Ca
2
+
Versus Mg
2
+
To many, there are insuf±cient fundamen-
tal differences between the pair of alkaline
earth metal ions Ca
2
+
and Mg
2
+
to ex-
plain their widely differing roles in biology.
There are, however, ample ways to ac-
count for the behavior differences between
Ca
2
+
and Mg
2
+
observed in biochemi-
cal systems.
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