Metal ions are very widespread in biological systems: some are present in enzymes, some in structural features like the calcium in bones, some in transport systems like the iron in haemoglobin, some in control systems like sodium and potassium in nerve cells. Generally speaking, metals of groups 1 and 2 (Na, K, Ca, Mg) tend to be present in fairly large quantities, whereas transition metals tend to be present in relatively small quantities, and are sometimes called trace metals. You can find a brief list of the roles of transition metals in biological systems here.

These metal ions are not usually present as free metal ions, but instead as complexes. The ligands involved may be a lot more complicated than the ones you are used to, but the donor atoms are the usual ones (O, N, S, very occasionally C) and the principles we have learnt in this course can be equally applied to them - things like matching the hard/soft properties of donor to metal, the effects of various geometries on orbital energies and the results of them, and so on.

Metals in biological systems are often bound to ligands called macrocycles - these are cyclic compounds that contain several donor atoms. Two of the most common examples are the porphyrin ring and the corrin ring:

Both of these compounds have four nitrogen donors. They are subtly different though: for one thing the porphyrin coordinates as a dianion, whereas the corrin coordinates as a mono-anion (the N's get deprotonated), and also the porphyrin is planar whereas the corrin tends not to be. Various substituents can be attached at those numbered positions (1-8, and the ones between the rings labelled with Greek letters) around the periphery of the macrocycle.

The most well known example of a porphyrin complex is the haem in haemoglobin, the oxygen-transporting protein in red blood cells. The way that haemoglobin works is shown schematically below:

Haemoglobin basically consists of two parts, the haem group (the iron with its porphyrin ligand), and a protein, globin. In its resting state with no oxygen bound, the iron is high spin Fe(II). When oxygen binds to make oxyhaemoglobin, it changes to low spin Fe(II). This, needless to say is a reversible process, so that when the red blood cell gets to an oxygen-deficient area, that equilibrium gets reversed and O₂ is released. Notice that this this is an example of oxygenation (adding oxygen) rather than oxidation, because the iron doesn't change oxidation state. Or at least not usually: if we do oxidise it to Fe(III) (and a very small proportion does actually do this for every oxygenation/deoxygenation cycle) we get a substance called methaemoglobin, which is inactive for oxygen transport. One of the functions of the globin protein is to stop this oxidation happening: if we disrupt the protein so that we separate the haem from the globin, we can isolate haem under strictly anaerobic conditions, but as soon as we expose it to air, it changes to Fe(III) in the form of haematin.

That change from high spin to low spin on oxygenation has far reaching consequences. In its high spin state, the radius of the iron atom is just too large to allow it to fit into the plane of thre porphyrin ring. But when we change to low spin, the radius decreases a bit (becasue all six electrons are now in the t_2g level) and it's thought this allows the iron to move into the plane - dragging with it its sixth ligand, a histidine group, and thus triggering off a cascade of conformational changes which affect other subunits of the haemoglobin. This results in cooperative binding of O₂: once an oxygen is bound to one subunit, the others pick one up more easily.

The best known example of a corrin ring is in Vitamin B12 cofactor, which contains a cobalt corrin complex:

The unusual thing about this species is that in the natural state the R group attached to cobalt is a methyl: this means that it's a rare example of a naturally-occurring organometallic compound (that's one with a direct metal-carbon bond).

As well as occurring in biogical systems, metal complexes can actually be used as drugs. The best example here is cis-platin, the name given to cis-[PtCl₂(NH₃)₂], which is used in cancer chemotherapy. This is a very simple complex, though there is one drawback: it has a certain degree of toxicity itself. The corresponding trans isomer has no beneficial effect, but is also toxic. Therefore it is important to be able to make the cis isomer in a pure form, uncontaminated by the trans. We can do this using a clever bit of chemistry called the trans effect, which you'll find out more about next year:

The trans effect is the ability of a ligand to labilise the ligand trans to itself, in other words it encourages the substitution reactions of that ligand. In this case the trans effect of chloride is greater than that of ammonia. So in the top diagram, once we have put one ammonia on, the second one goes on trans to one of the remaining chlorides to give us exclusively the cis isomer. In the bottom diagram, if we start from [Pt(NH₃)₄]²⁺, when we put one chloride on, the second will go on trans to the first, resulting in the trans isomer. So we can make both isomers in a controlled way.

Vitamin B12 is an important coordination compound inbiology.  It is an interesting biomolecule in the sense thatno other vitamin contains a metal ion.  This is the onlynaturally occurring organometallic compound found inbiology.  An intriguing aspect of vitamin B12 is the greatstability of the metal-carbon bond.  A great deal of new andinteresting inorganic chemistry has been uncovered whilestudying systems pertinent to B12.  In this article somesalient features of this unique molecule (B12) and its modelcompounds are presented.Vitamin B12 is one of the naturally occurring coordinationcompounds in biology.  Some of the other important examplesare chlorophyll, haemoglobin, myoglobin and cytochromes.  Thecommon feature in these biomolecules is that a metal ion isenclosed in a macrocyclic ligand.  Although vitamin B12 iscertainly the most complex non-polymeric compound found innature, it is devoid of protein structure making its biological rolerelatively easy to understand.Vitamin B12 is known to be present in plants, animals and alsoin bacteria.  It plays an important role in the metabolism ofnucleic acids and in protein synthesis.  It is of criticali m p o r t a n c e   i n   t h e   r e a c t i o n   b y   w h i c h   r e s i d u e s   f r o mcarbohydrates, fats and proteins are used to produce energy inliving cells. In humans, deficiency of vitamin B12 leads topernicious anaemia.  In this article, the structure and reactivityof vitamin B12 and its analogues are described.  Attempts tomodel the complex biomolecule using simple coordinatingcompounds are discussed.

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