Chemistry Reference
In-Depth Information
However, as we mentioned earlier, there is a second, and absolutely indispensable criterion for selection
e
namely that the element must fulfil a function which is both an absolute requirement for life as it existed at that
moment in time, and which cannot, or may not, be fulfilled by some other element. We can immediately see that
the six elements involved in the earlier definition of organic chemistry are ideally placed to do their job, forming
covalent bonds with tetravalent carbon. We will return to the case of chlorine later, but what of the four metal ions
in the top eleven?
One interesting way to compare the suitability for function with the properties of a number of key selected
TABLE 1.1
Correlations Between Ligand Binding, Mobility, and Function of some Biologically Relevant
Metal Ions
Metal Ion
Binding
Mobility
Function
Na
þ
,K
þ
Weak
High
Charge carriers
Mg
2þ
,Ca
2þ
Moderate
Semi-mobile
Triggers, transfers structural
Zn
2þ
Moderate/Strong
Intermediate
Lewis acid, transfers structural
Co, Cu, Fe, Mn, Mo
*
Strong
Low
Redox catalysts Oxygen chemistry
*Charge not given, since this varies with oxidation state
atom, group, or molecule that is attached to the central metal ion), the mobility, and the functions of a number of
important biologically relevant metal ions. What emerges immediately is that as the strength of binding of the
metal ion to biological ligands decreases, the mobility of the metal ion increases, and it is therefore able to
function much more effectively as a transporter of charge. Thus, Na
þ
and K
þ
(together with H
þ
and Cl
), which
bind weakly to organic ligands, are ideally suited to generate ionic gradients across biological membranes, and to
ensure the maintenance of osmotic balance. This is precisely what these two essential alkali metal ions do in
biological systems, although, as we will see in Chapter 9, they also have other interesting additional roles. In
contrast, Mg
2
þ
and Ca
2
þ
, with intermediate binding strengths to organic ligands, can play important structural
roles and, particularly in the case of Ca
2
þ
, serve as a charge carrier and a trigger for signal transmission within the
cell. The various roles of these two alkaline earth cations are discussed respectively in Chapters 10 and 11.
The inclusion of the six transition metal ions
cobalt, copper, iron, manganese, molybdenum, and zinc
in
e
e
Table 1.1
is no coincidence
we saw earlier that they are essential trace elements for man. So, together with the
11 bulk elements, we have now identified 17 of the 'essential' elements. Their relative positions in the Periodic
Table are shown in
Fig. 1.3
,
which presents the first six rows of the Periodic Table, colour-coded into families.
Zn
2
þ
has ligand-binding constants intermediate between those of Mg
2
þ
and Ca
2
þ
and those of the group of five
other transition metals. Unlike them, zinc effectively does not have access to any other oxidation state than Zn
2
þ
(the
e
1 state compounds are very unstable). Zn
2
þ
not only plays a structural role, but can also fulfil a very
important function as a Lewis acid (Chapter 12).
The other five transition metal ions, Co, Cu, Fe, Mn, and Mo, bind tightly to organic ligands and participate in
innumerable redox reactions. Fe and Cu are constituents of a large number of proteins involved in electron-transfer
chains. They also play an important role in oxygen-binding proteins involved in oxygen activation, as well as in
oxygen transport and storage (Chapters 13 and 14). Co, together with another essential transition metal, Ni, was
particularly important in the metabolism of small molecules like carbon monoxide, hydrogen, and methane, which
were thought to be abundant in the reducing atmosphere of early evolution, and is still utilised by a number of
microorganisms (Chapter 15). Although Co is an essential element for man, Ni proteins are virtually unheard of in
þ
