Applications & Special Topics
The last part answers the question a student is entitled to ask after eight parts of theory: what is any of this for. Haemoglobin is a coordination compound whose behaviour is a spin-state change. Cisplatin is a substitution reaction with a trans-effect synthesis and a kinetic-inertness rationale. An industrial catalytic cycle is oxidative addition and reductive elimination in a loop. Each application here is chosen because it uses something the earlier parts built, and each is traced back to it. The part closes with the master index to all nine. Two layers on every section: a slow, hand-held beginner path and a research-grade advanced/reference path.
The 12 sections in Part 9
- 1L.1 Oxygen transport and storage — a spin-state change with consequences Free below
- 2L.2 Other metalloproteins read as coordination compounds
- 3L.3 Metallodrugs — cisplatin, contrast agents and chelation therapy
- 4L.4 The elementary steps, with the electron count tracked throughout
- 5L.5 Three catalytic cycles, worked vertex by vertex
- 6L.6 Metal–metal bonds, the δ bond, and clusters
- 7L.7 Crown ethers, cryptands and size-match selectivity
- 8L.8 Self-assembly and metal–organic frameworks
- 9L.9 Molecular recognition — and where the subject goes next
- 10Section-by-section index
- 11The argument, from Part 1 to Part 9
- 12The nine parts
L.1 Oxygen transport and storage — a spin-state change with consequences
Free extractSection 1 of Part 9, reproduced in full from the book — figures and all. No sign-in, no paywall on this section.
Haem, myoglobin and haemoglobin, told as a story you could repeat to someone else. The chemistry is Part 3’s high-spin/low-spin distinction with a protein wrapped round it.
Oxygen is not very soluble in water. A litre of blood plasma dissolves only a few millilitres of it — nowhere near enough to run a mammal. Evolution’s answer was to build a reversible O₂ ligand-binding site, and the site it built is an iron(II) complex.
The ligand: protoporphyrin IX
Haem is iron(II) bound to protoporphyrin IX, a tetradentate macrocyclic ligand. Three of the descriptors this book has developed apply at once, and they are worth naming explicitly because a question can ask for any of them:
- Denticity 4 (Part 1) — four pyrrole nitrogens, all N-donors, all in one plane.
- Charge 2− — the free base loses two NH protons on metallation, so the ligand carries a 2− charge and Fe(II)–porphyrin is neutral overall. This is why haem is not simply washed out of the cell.
- Macrocyclic (Part 7) — the four donors are pre-organised in a ring, so the complex enjoys the macrocyclic effect on top of the ordinary chelate effect. A macrocycle does not have to pay the conformational entropy cost of wrapping itself round the metal, because it is already wrapped. That is precisely why nature uses a macrocycle for a site that must never let go of its iron.
The five-coordinate trick
A metal ion bound by four planar donors has two axial positions left. In myoglobin and haemoglobin, one of them is filled permanently by an imidazole nitrogen from a histidine residue of the protein — the proximal histidine. The other is left empty. That vacant sixth site is the oxygen-binding site.
Distal histidine: a second histidine on the O₂ side, not coordinated to iron. It hydrogen-bonds bound O₂ and sterically discriminates against CO.
What happens on binding — the whole thing in one sentence
Five-coordinate high-spin iron(II), too big for the porphyrin hole and therefore sitting out of plane, binds O₂ at the vacant sixth site; the extra strong-field ligand converts it to six-coordinate low-spin iron(II), which is smaller and drops into the plane, dragging the proximal histidine — and with it the protein — along for the ride.
Read that figure against Part 3 and it is entirely familiar. High-spin d⁶ puts two electrons into the eg set, which points straight at the ligands and is antibonding: metal–ligand distances are long and the effective ionic radius is large. Low-spin d⁶ empties the eg set completely, giving the t2g⁶ closed shell — the most stabilised configuration in the whole octahedral CFSE table, with CFSE = −2.4Δo + 2P. The ion contracts, and now it fits.
The magnetic consequence is measurable and is the classic experimental proof: deoxyhaemoglobin is paramagnetic and oxyhaemoglobin is diamagnetic. That single observation, which is Part 6 chemistry, is what pinned down the spin-state change in the first place, and it is why functional MRI can see oxygenated versus deoxygenated blood at all — the BOLD contrast mechanism is literally a paramagnetism measurement.
One haem is a store; four haems are a transport system
Myoglobin has one polypeptide chain and one haem. It sits in muscle and holds oxygen until the muscle needs it. Binding is simple 1:1 equilibrium, so its saturation curve is hyperbolic and it is nearly full even at the low oxygen pressures found in tissue — exactly the behaviour of a store.
Haemoglobin has four chains (α₂β₂) and four haems. Its job is different: pick up oxygen in the lungs at high pO₂, release it in tissue at low pO₂, and do so efficiently — i.e. change its saturation a lot for a modest change in pressure. A hyperbolic curve cannot do that. A sigmoid one can.
The mechanism is the figure you have already seen. When the first O₂ binds to one subunit, that subunit’s iron drops into its porphyrin plane, pulls its proximal histidine, and shifts the helix the histidine belongs to. Those helix movements are transmitted across the subunit interfaces, and they make it easier for the other three subunits to undergo the same change. The whole tetramer switches between two quaternary states:
| T state (tense) | R state (relaxed) | |
|---|---|---|
| Oxygen affinity | Low | High |
| Dominant when | Deoxygenated — tissue | Oxygenated — lung |
| Iron position | Out of plane, high-spin, ring domed | In plane, low-spin, ring flat |
| Subunit interfaces | More salt bridges, constrained | Salt bridges broken, freer |
| Stabilised by | H⁺, CO₂, Cl⁻, 2,3-BPG | Bound O₂ itself |
The Hill coefficient — putting a number on cooperativity
Plot log[Y/(1−Y)] against log p and the slope is the Hill coefficient n. Read it as follows:
- n = 1 — independent, non-cooperative sites. Myoglobin.
- n > 1 — positive cooperativity. Haemoglobin, n ≈ 2.8 at the midpoint.
- n < 1 — negative cooperativity: binding one ligand makes the next harder.
The Bohr effect
Chemically it is a coupled equilibrium. Protons and CO₂ (the latter partly as carbamate on N-terminal amino groups) bind preferentially to the T state, stabilising it. Stabilising T means shifting the T⇋R equilibrium away from the high-affinity state, which lowers oxygen affinity and shifts the saturation curve to the right. The organic phosphate 2,3-bisphosphoglycerate does the same thing more powerfully still, binding in the central cavity of the T-state tetramer.
Note the elegance from a coordination-chemistry point of view: the effector molecules never touch the metal. They act entirely by shifting a conformational equilibrium that in turn shifts a ligand-field equilibrium. Allostery is thermodynamic linkage, not chemistry at the metal.
Why carbon monoxide kills
CO is isoelectronic with N₂ and CN⁻, and Part 4 established what that means: it is a strong σ-donor and a good π-acceptor, high in the spectrochemical series, and it stabilises low oxidation states through back-donation. It binds Fe(II) very much harder than O₂ does. Bound CO occupies the very site oxygen needs, and worse: a partly carbonylated haemoglobin is locked towards the R state, so the remaining subunits bind their oxygen too tightly to release it in tissue. Carbon monoxide poisoning is therefore doubly damaging — it removes capacity and sabotages delivery of what capacity remains.
⚠ Common mistakes & exam traps
- Do not say the iron is oxidised on binding O₂. The functional cycle is Fe(II)⇋Fe(II). Iron(III) haemoglobin — methaemoglobin — does not bind O₂ and is physiologically useless; the body maintains a reductase specifically to reverse its formation. (The honest subtlety in the electronic description is discussed in the advanced layer below.)
- The Hill coefficient is not the number of subunits. n ≈ 2.8 for a four-subunit protein.
- Myoglobin is a store, not a transporter. It cannot be cooperative: cooperativity requires more than one interacting site, and myoglobin has one.
- The distal histidine is not coordinated to iron. Only the proximal one is. Writing haemoglobin as a bis(imidazole) complex is wrong — it would leave nowhere for O₂ to bind.
- The Bohr effect shifts the curve right, not down. The maximum saturation is unchanged; what changes is the pressure needed to achieve it.
Deoxyhaemoglobin has a measured magnetic moment of about 5.4 μB per haem, oxyhaemoglobin essentially zero. Deduce the spin states and comment. Medium
The electronic description of bound O₂ — where the accepted picture is firm and where it is still argued; and the quantitative shape of cooperativity.
What exactly is bound: Fe(II)–O₂, or Fe(III)–superoxide?
This is the one place in L.1 where the honest answer is more interesting than the textbook one, and where a careless statement in an exam script is genuinely wrong rather than merely incomplete.
Three descriptions have been proposed for the Fe–O₂ unit in oxyhaemoglobin:
| Model | Formal description | Predicted spin | Status |
|---|---|---|---|
| Pauling | Fe(II) (low-spin, d⁶) with neutral O₂ bound end-on and bent | Diamagnetic — both partners closed-shell in the bound state | The description almost universally taught, and the one to give unless a question explicitly asks for more |
| Weiss | Fe(III) (low-spin, d⁵, S = ½) bound to superoxide O₂⁻ (S = ½) | Diamagnetic only if the two spins are antiferromagnetically coupled to a singlet | Strongly supported by spectroscopic and computational evidence for substantial charge transfer from iron to O₂ |
| McClure–Goddard (ozone model) | A more covalent picture in which neither integer assignment is right; the Fe–O–O unit is treated as a single delocalised entity | Diamagnetic | A serious third position in the literature |
What is not in dispute, and is what an examiner is testing:
- The complex is diamagnetic, S = 0 overall.
- O₂ binds end-on and bent (η¹, Fe–O–O angle near 120°), not side-on and not linear.
- There is substantial charge transfer from iron towards O₂: the O–O stretching frequency of bound dioxygen falls into the range characteristic of superoxide, well below that of free O₂. The bound ligand really does carry significant negative charge.
- Reversibility requires that the charge transfer not go to completion. If genuine Fe(III) and free superoxide were released, the protein would auto-oxidise to methaemoglobin on every breath.
How to answer. Say: “formally described as low-spin Fe(II) bound to O₂, but with substantial Fe→O₂ charge transfer, so the unit has appreciable Fe(III)–superoxide character; the complex is diamagnetic either way, and the O–O stretching frequency supports the superoxide contribution.” That sentence is correct under any of the three models and shows you know why the question exists.
Picket-fence porphyrins — the control experiment
A simple iron(II) porphyrin in solution does not reversibly bind O₂: two of them meet, form a μ-peroxo bridge Fe–O–O–Fe, and go on to an irreversibly oxidised μ-oxo dimer Fe–O–Fe. The protein prevents this simply by burying each haem in its own hydrophobic pocket so that no two irons can meet.
Collman’s picket-fence porphyrins reproduce this synthetically: bulky amide “pickets” on one face of the porphyrin create a protected cavity, a hindered base occupies the other axial site, and the resulting complex binds O₂ reversibly in ordinary solution. It is the clean demonstration that the protein contributes steric protection and a hydrophobic environment, not any special electronic magic — the coordination chemistry is doing the work.
On oxygen binding and cooperativity.
- M. F. Perutz, ‘Stereochemistry of cooperative effects in haemoglobin’, Nature, 1970 — the structural origin of the T→R transition. Volume and page numbers to be confirmed.
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Read the rest of Part 9
The remaining 11 sections of this part — L.2 Other metalloproteins read as coordination compounds, L.3 Metallodrugs — cisplatin, contrast agents and chelation therapy, L.4 The elementary steps, with the electron count tracked… — and all nine parts of Coordination Chemistry are part of ChemVidya Full Access, along with the other books, 55 Study Notes and 6,000+ practice questions.
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