654
Bioinorganic Chemistry
However, by allowing a greater diversity of
energy-producing reactions, the increasing
dioxygen concentration promoted the rise
of eukaryotes about 1 500 million years ago
and of the Frst multicellular organisms
about 650 million years ago.
Prokaryotes do not form true multicellu-
lar structures. Later, sunlight interacting
with dioxygen made enough ozone in
the stratosphere to provide an umbrella
for ultraviolet radiation, so that about
410 million years ago, plants were able
to invade the land. Thus, though life ap-
peared at just about the Frst 1 000 million
years of the earth’s history, it took over
3 000 million years from the origin of
life on earth through the production of
a dioxygen-containing atmosphere and
ozone shield to extensive land plants.
Dioxygen is the pivotal element the ab-
sence or presence of which determines the
forms of life on earth. It took more than
2 000 million years after the Frst appear-
ance of dioxygen in the atmosphere to yield
an environment in which multicellular or-
ganisms arose and became the basis for
life as we know it today.
Dioxygen, so necessary for the life forms
on earth today, is also toxic and contributes
to cell aging. With two unpaired spins, O
2
reacts readily with radicals and some metal
ion complexes. Subsequent reactions gen-
erate other reactive molecules and radicals:
hydrogen peroxide, H
2
O
2
; hydroperox-
ides, ROOH; superoxide, O
2
;an
dth
e
hydroxyl radical, HO, which is highly
damaging. The last two species initiate
deleterious radical chain lipid peroxida-
tions that contribute to aging. Therefore,
pivotal dioxygen provides life with both
the source of its versatility via oxidation
processes and the seeds of its demise by
radical reactions.
Almost all organisms possess an en-
zyme, superoxide dismutase, to rid cells of
readily generated and reactive superoxide,
O
2
, by converting it to less-toxic hydro-
gen peroxide, 2 O
2
+
2H
+
H
2
O
2
+
O
2
. Superoxide dismutase is much more
widespread than catalase, which catalyzes
the decomposition of hydrogen peroxide.
15
Metal Ion Complexation Kinetics
Belying a common understanding of the
word
formation
that might imply an addi-
tion reaction, metal ion complex formation
in solution is almost always a
substitution
(occasionally with a reduction in coordina-
tion number). In aqueous systems, bound
water undergoes substitution either in the
reactant complex or as an intermediate in
the route from reactants to products.
15.1
Substitution Reactions
The greater complexity of kinetics over
thermodynamics is shown in the simple-
appearing ligand replacement reaction
with
n
Fxed ligand donor atoms, L, bound
to metal ion M (charges not speciFed).
L
n
MX
+
Y
−−−→
L
n
MY
+
X
While thermodynamics deals with the
species above, kinetics also includes addi-
tional species involved in the mechanism
of the reaction. Substitution of uniden-
tate ligand X by unidentate ligand Y may
proceed by a reduction of coordination
number (dissociation) to give the interme-
diate L
n
M, by an increase in coordination
number (association) to yield the interme-
diate L
n
M(X)Y, or by a process in which
the incoming and outgoing ligands inter-
change in a more or less concerted fashion.
±rom the wholly dissociative mechanism
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