Chemistry Reference
In-Depth Information
Mo (typically thermophilic Bacteria and hyperthermophilic Archae
9
)
. Walso replaced osmium and tantalum (and
before that, carbon) in the electric lamps of the early twentieth century, which gradually replaced gas lamps.
Tungsten lamps, made of tightly coiled helices of finely drawn tungsten wire, in bulbs filled with argon, would
provide, according to Edison's vision, 'light for the masses', thereby conquering the fear of darkness. As Oliver
Sacks delightfully recounts in his wonderful book 'Uncle Tungsten', the history of chemical discovery has been
inseparable from the quest for light (
Sacks, 2001
,
p. 337).
The next six transition metals, Mn, Fe, Co, Ni, Cu, and Zn, are all essential and are discussed in detail in later
chapters. While it is important as an oxygen atom donor, the most significant contribution to biology of Mn, the
first of the group 7 elements, is the catalytic cluster involved in the photosynthetic oxidation of water in plants, and,
at a much earlier point in geological time, in cyanobacteria (Chapter 16). This reaction generates oxygen, which,
of course, changed the whole pattern of the evolution of planet Earth. Perhaps the greatest pollution event in the
history of our planet, this progressively moved us from an essentially reducing atmosphere to the oxidative world
that we now know. There were, of course, advantages
respiration is almost 20 times more effective at producing
ATP than fermentation, but with that came the disadvantages associated with the generation of highly reactive, and
toxic oxygen species (often designated the oxygen paradox). Neither of the other two group 7 elements have
biological roles. Technetium (Tc), like Sc, filled a gap in Mendeleev's periodic table, and many of its properties
were predicted by him before its discovery only in 1937. Tc has no stable isotopes
e
all of its isotopes are
radioactive. Almost all Tc is produced artificially (only minute amounts are found in nature), and since it was the
first of the predominantly artificial elements, it was given the name technetium (from the Greek
e
s
3cnh
s
o
ยด
2
e
artificial). The short-lived isotope of Tc, technetium-99 m (half-life 52.5 min), renders invaluable service in
nuclear medicine as a tracer for in vivo imaging by positron emission tomography. Rhenium, Re, is one of the
rarest elements in the Earth's crust, and is among the most expensive industrial metals (over 6000$/kg in 2009). Jet
engines, which contain up to 6% of Re, represent its largest use, with industrial catalysts the next most important.
The oxygen paradox, referred to in the previous paragraph, results from the capacity of iron and copper to
generate toxic reactive oxygen species, notably the hydroxyl radical through the well-known Fenton reaction. The
many and diverse roles that Fe can play will be presented in more detail in Chapter 13. They include oxygen
carriers, enzymes that activate and insert oxygen into a wide variety of substrates, electron-transfer proteins (both
haem-containing cytochromes and iron-sulfur proteins), as well as iron-storage and transport proteins, to name but
a few. With very few exceptions, Fe is essential for almost all living organisms, most probably because of its role
in the generation of the amino acid radicals required for the conversion of ribonucleotides to deoxyribonucleo-
tides, essential for DNA synthesis, by Fe-dependent ribonucleotide reductases. Ru, like Rh and Pd (see below), has
important applications in industrial catalysis. Organometallic ruthenium complexes are highly efficient catalysts
for olefin metathesis with important applications in organic and pharmaceutical chemistry, while other ruthenium
complexes are increasingly being used in solar energy technologies. More recently, Ru-based drugs have been
developed as anticancer agents. Osmium is the densest natural element, and its tetroxide is widely used for
staining biological tissue for electron microscopy.
Co, although present at much lower levels in mammals than Zn, Fe or Cu, is nonetheless indispensable in
a number of important vitamin B
12
-dependent enzymes, including the ribonucleotide reductases of organisms, like
Lactobacilli,
10
which do not have access to iron. The ribonucleotide reductases of these organisms use a cobalt-
based cofactor, related to vitamin B
12
. Co is also used in a number of other enzymes, some of which catalyse
complex isomerisation reactions. Neither Rh nor Ir, the other members of group 10, has any biological impli-
cations. As much as 80% of the world production of rhodium goes into catalytic converters, which convert up to
90% of harmful gases from auto exhaust (hydrocarbons, carbon monoxide, and nitrogen oxide) into less harmful
substances (nitrogen, carbon dioxide, and water vapour).
,
9. The three primary kingdoms of cells are Eukaryotes, Bacteria, and Archae.
10. So called since they are found in milk, where the iron-binding protein lactoferrin, sequesters iron so tightly that it is no longer available for
microbial requirements.
