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Spec Mapping — OCR H432 Module 5.3.1 — Transition elements (cisplatin and haemoglobin), covering the structure and clinical use of cisplatin (cis-diamminedichloroplatinum(II), square-planar d⁸ Pt(II)) as an anticancer drug; the aquation–DNA-binding mechanism by which cisplatin cross-links the N7 atoms of adjacent guanines and triggers apoptosis; the importance of the cis geometry (trans-platin is therapeutically inactive); ethical and clinical issues including side effects (nephrotoxicity, ototoxicity, peripheral neuropathy) and second-generation derivatives (carboplatin, oxaliplatin); the structure of haem as an octahedral Fe(II) complex with tetradentate porphyrin + proximal histidine + variable sixth ligand (O₂, CO, H₂O); cooperative reversible O₂ binding via the T→R quaternary-structure transition; and the mechanism of CO toxicity (CO binds Fe(II) ~200× more strongly than O₂, forming an effectively irreversible carboxyhaemoglobin complex) (refer to the official OCR H432 specification document for exact wording).
Cisplatin and haemoglobin are the two named biological complex ions in the OCR H432 specification, and together they illustrate the entire conceptual range of transition-metal coordination chemistry as it appears in living systems. Cisplatin is a synthetic anticancer drug whose discovery by Barnett Rosenberg in 1965 (initially serendipitously, while studying the effect of electric fields on bacterial cell growth) transformed the prognosis for testicular cancer from near-uniformly fatal to a 90%+ cure rate; haemoglobin is the natural oxygen-transport protein in red blood cells whose structure was solved by Max Perutz and John Kendrew (Nobel Prize 1962) over three decades of X-ray crystallography. The two cases instantiate four core A-level themes: (i) the distinction between square-planar d⁸ geometry (cisplatin, Pt²⁺) and octahedral d⁶ geometry (haemoglobin, Fe²⁺); (ii) the role of cis-trans isomerism in determining biological activity (cisplatin works, trans-platin does not); (iii) the importance of ligand-substitution kinetics (cisplatin aquates inside cells before binding DNA); and (iv) the mechanism of competitive ligand binding (CO outcompeting O₂ for the haem iron). This lesson works through each in turn with the structural detail, mechanistic logic and tiered specimen-band answers that OCR examiners expect.
Key Definition — Cisplatin ([Pt(NH₃)₂Cl₂], cis-diamminedichloroplatinum(II)) is a square-planar d⁸ platinum(II) coordination complex in which two ammine and two chloride ligands occupy the four corners of a planar square around the Pt(II) centre in the cis (adjacent) arrangement. Its anticancer activity depends on aquation inside cells (loss of one or both chlorides as the intracellular Cl⁻ concentration is much lower than extracellular), followed by binding of the resulting Pt(NH₃)₂(H₂O)ₓ to the N7 atoms of two adjacent guanine bases on the same DNA strand to form a 1,2-intrastrand cross-link, which kinks the DNA and triggers apoptosis. The trans isomer is therapeutically inactive because its two binding sites are 180° apart and cannot reach two adjacent bases.
Key Definition — Haemoglobin is the iron-containing oxygen-transport protein in vertebrate red blood cells; each tetrameric haemoglobin molecule contains four haem prosthetic groups, each comprising an Fe²⁺ ion in an octahedral coordination sphere: four nitrogen atoms from a tetradentate porphyrin ring lie in the equatorial plane, a fifth coordination site is occupied by an imidazole nitrogen from the proximal histidine (His-F8) of the globin protein, and a sixth coordination site reversibly binds O₂ in the lungs and releases it in the tissues. Carbon monoxide can bind to the same site ~200× more strongly than O₂, forming an effectively irreversible carboxyhaemoglobin complex and causing CO toxicity by competitive ligand binding.
By the end of this lesson you should be able to:
Cisplatin is the common name for cis-diamminedichloroplatinum(II), formula [Pt(NH₃)₂Cl₂]. It is one of the most widely used chemotherapy drugs in modern medicine and has dramatically improved survival rates for testicular cancer (cure rate now >90%), ovarian, bladder, cervical, lung (especially small-cell), head-and-neck, and some childhood cancers since it was approved by the US FDA in 1978. The story of its discovery is one of the great accidents of modern chemistry: Barnett Rosenberg at Michigan State University was studying whether electric fields might affect bacterial cell division using platinum electrodes in a culture medium containing ammonium chloride. He observed that bacterial growth was inhibited near the electrodes — but the cause turned out to be not the electric field but a tiny amount of platinum that had electrolysed off the electrodes and combined with the ammonia and chloride in solution to give cis-Pt(NH₃)₂Cl₂. Rosenberg recognised this as a potential anticancer agent and the molecule entered clinical trials in 1971.
Platinum(II) is a d⁸ ion, and like all d⁸ metals (Pd²⁺, Pt²⁺, Au³⁺, Ni²⁺ in some complexes) it preferentially forms square-planar complexes rather than tetrahedral ones. The reason is crystal-field stabilisation: in a square-planar d⁸ geometry the eight d-electrons fill the four lower-energy d-orbitals (dxz, dyz, dxy, dz²) completely, leaving the high-energy dx²−y² (which points directly at the four ligands) empty — a particularly stable arrangement. Four ligands sit at the corners of a square around the Pt²⁺ centre, with adjacent L–Pt–L bond angles of 90° and opposite L–Pt–L bond angles of 180°. With two NH₃ and two Cl⁻, there are two possible geometric arrangements:
The difference between the two isomers is purely geometric — they have the same molecular formula, the same ligands, the same overall charge (neutral, 0), and the same oxidation state at Pt (+2) — but their three-dimensional arrangement of ligands gives them dramatically different physical and biological properties. The cis isomer has a measurable dipole moment (because the two electronegative Cl⁻ sit on the same side of the molecule); the trans isomer has zero net dipole moment (the two Pt–Cl bond vectors cancel by symmetry). The cis isomer is more water-soluble (the dipole assists hydration) and is what reaches cancer cells from an intravenous infusion. But the decisive difference is biological, not chemical — only the cis isomer can form the bidentate DNA cross-link that gives cisplatin its anticancer mechanism.
Cisplatin is administered as an intravenous infusion in saline (0.9% NaCl) solution. In the bloodstream the extracellular Cl⁻ concentration is about 100 mM, high enough to suppress aquation of the drug — the two chloride ligands stay coordinated to Pt and the drug remains as neutral [Pt(NH₃)₂Cl₂], which is its stable transport form. Once cisplatin diffuses across the cell membrane into the cytoplasm, however, the intracellular chloride concentration is only ~4 mM — about 25-fold lower than outside the cell. This drop in [Cl⁻] shifts the substitution equilibrium so that water (an enormously abundant ligand at ~55 M intracellularly) replaces one or both chlorides:
mathrm[Pt(NH3)2Cl2]+H2Orightarrow[Pt(NH3)2Cl(H2O)]++Cl−
mathrm[Pt(NH3)2Cl(H2O)]++H2Orightarrow[Pt(NH3)2(H2O)2]2++Cl−
The aquated species — cis-[Pt(NH₃)₂Cl(H₂O)]⁺ and cis-[Pt(NH₃)₂(H₂O)₂]²⁺ — are positively charged and far more reactive towards nucleophiles than the parent dichloride. Water is a relatively weak ligand (it sits low in the spectrochemical series — see lesson 5), so it is readily displaced by the much stronger N-donor sites on DNA. The Pt(II) centre seeks out and binds preferentially to the N7 atom of guanine in the DNA major groove — a nitrogen with a lone pair that is not involved in Watson–Crick base pairing and is therefore sterically accessible. Adenine N7 can also be targeted, though less frequently.
Crucially, cisplatin forms a bidentate adduct in which the Pt(II) bridges two adjacent guanines on the same DNA strand (a 1,2-intrastrand cross-link, the dominant adduct, ~65% of all platinum–DNA adducts; the remaining ~25% are 1,2-intrastrand AG cross-links and ~5% are 1,3-intrastrand GNG cross-links with one base between the platinated guanines):
mathrmcistext−[Pt(NH3)2(H2O)2]2++2,G(DNA)rightarrowcistext−[Pt(NH3)2(G)2]+2,H2O
The geometry of this bidentate adduct kinks the DNA helix by approximately 35–60°, severely distorting the local double-helical structure. The damaged DNA cannot be properly replicated (DNA polymerase stalls at the lesion) or transcribed (RNA polymerase stalls), and the cell's repair machinery (nucleotide excision repair, NER) recognises the lesion but in tumour cells often cannot fix it before the damage signal becomes overwhelming. The accumulated genomic stress triggers apoptosis — programmed cell death via the p53 / caspase pathways. Rapidly dividing cells — including cancer cells (which divide much faster than most normal tissues), but also hair follicles, gut-lining epithelium, and bone-marrow progenitors — are the most vulnerable because they encounter the platinum lesions during S-phase DNA synthesis and trigger apoptosis at the highest rate.
The trans isomer of [Pt(NH₃)₂Cl₂] is therapeutically inactive. The reason is purely geometric: in transplatin, the two chloride ligands (and therefore the two sites that can be aquated and replaced by guanine donors) are 180° apart, on opposite sides of the square-planar Pt centre. After both chlorides aquate and one guanine binds at one site, the second guanine would have to reach all the way across the Pt centre to bind at the other site — a distance of roughly 460 pm. Adjacent guanines in B-form DNA have their N7 atoms only ~320 pm apart, and even allowing for some flexibility in the DNA backbone, the geometry simply cannot accommodate a trans-Pt bridge to two adjacent purines on the same strand.
The cis isomer has its two binding sites only ~280 pm apart (the cis Cl–Pt–Cl distance), well within the reach of two adjacent guanine N7 atoms. The two NH₃ ligands stay coordinated throughout and provide the structural framework for the Pt(II) centre, while the two cis-aquated sites serve as the "biting" claws that grip the DNA.
This geometric argument is the single most-examined point in OCR Year-13 transition-metal chemistry: the cis geometry is essential because the two binding sites must be close enough together to bridge two adjacent guanines on the same DNA strand. Trans-platin can still aquate (it loses Cl⁻ for H₂O at a similar rate to cisplatin), and the trans aquated species can still bind to one guanine. But it can only form a monodentate adduct, which is much less damaging to the DNA structure and is more easily repaired by the cell. So trans-platin is essentially harmless at therapeutic concentrations — both to cancer cells and to healthy cells.
Cisplatin is an extraordinarily effective drug — testicular cancer cure rates went from <10% before cisplatin to >90% after — but its mechanism is non-specific and it kills rapidly-dividing healthy cells as well as cancer cells. The major side effects are:
Because cisplatin kills all rapidly-dividing cells indiscriminately, it is a "blunt instrument" of cancer treatment. Newer second-generation platinum drugs are designed to retain anticancer activity while reducing toxicity:
Drug resistance is a clinical problem: some tumours develop or acquire mechanisms that reduce cisplatin effectiveness — decreased cellular uptake, increased efflux by P-glycoprotein pumps, increased intracellular glutathione (which quenches the reactive Pt(II) before it reaches DNA), and enhanced DNA repair. Combination therapy (cisplatin + other cytotoxic agents) and the use of second-generation derivatives can sometimes overcome resistance.
Ethical considerations in clinical practice include: (i) informed consent — patients must understand the trade-off between potential cure and severe, sometimes permanent side effects; (ii) equity of access — second-generation derivatives are more expensive and not universally available, especially in low-income countries; (iii) animal testing — cisplatin's development relied on extensive animal studies, raising ongoing ethical debate; (iv) paediatric use — the lifelong consequences (hearing loss, neuropathy, fertility, secondary cancer risk) are particularly severe for children, requiring careful risk-benefit assessment.
Haemoglobin (Hb) is the iron-containing oxygen-transport protein in vertebrate red blood cells. Each Hb molecule is a tetramer of four globin protein subunits — two α-chains and two β-chains — and each subunit harbours one haem prosthetic group at its core. So one haemoglobin tetramer can carry up to four O₂ molecules simultaneously, one per haem. The atomic structure of haemoglobin was solved in 1959 by Max Perutz at the MRC Laboratory of Molecular Biology in Cambridge using X-ray crystallography — work that earned him the 1962 Nobel Prize in Chemistry (shared with John Kendrew, who solved the related muscle-oxygen protein myoglobin). Perutz's structure was the first ever of a protein and revolutionised our understanding of how molecular structure encodes biological function.
The haem group is a flat, planar organic molecule centred on a tetradentate macrocyclic ligand called a porphyrin ring (specifically protoporphyrin IX in haemoglobin). The porphyrin consists of four pyrrole rings linked by methine bridges to form a square macrocycle, with four nitrogen atoms pointing inward to the centre. At this centre sits a single Fe²⁺ ion coordinated to all four pyrrolic nitrogens by dative covalent bonds. The porphyrin is therefore a tetradentate ligand — it provides four bonds to the Fe²⁺ in a roughly square-planar equatorial arrangement.
The Fe²⁺ in haemoglobin is, however, octahedrally coordinated — there are two additional coordination sites perpendicular to the porphyrin plane:
The total coordination number of Fe²⁺ in oxy-haemoglobin is therefore 6 (4 porphyrin N + 1 histidine N + 1 O₂), and the geometry is octahedral. A second histidine (the distal histidine, His-E7) sits above the sixth coordination site but does not coordinate directly to Fe — instead it hydrogen-bonds to the bound O₂ (or H₂O) and helps stabilise the bent Fe–O–O geometry while destabilising the linear Fe–C–O geometry of bound CO, which is part of why O₂ binds reasonably tightly and CO binds even more tightly but not as much tighter as the intrinsic affinity would suggest.
graph TD
A[Haem Fe II center] --> B["4 nitrogens of porphyrin ring<br/>tetradentate equatorial"]
A --> C["1 nitrogen of proximal histidine F8<br/>from globin protein"]
A --> D["1 distal site variable<br/>O2 CO H2O or empty"]
B --> E["Coordination number 6<br/>octahedral geometry"]
C --> E
D --> E
D --> F["distal histidine E7<br/>not coordinated<br/>H-bonds to O2"]
The Fe in oxyhaemoglobin remains formally Fe(II) (+2 oxidation state) even when O₂ is bound. This is critically important: if the iron were oxidised to Fe(III) — the resulting protein is called methaemoglobin (met-Hb) — it would no longer bind O₂ at all, and the carrier function would be lost. The local environment of the haem pocket (the hydrophobic surroundings, the distal histidine H-bond, the bent Fe–O–O geometry) protects the bound O₂ from auto-oxidising the Fe²⁺ to Fe³⁺. About 1% of haemoglobin in red cells is met-Hb at any time, and a specific enzyme (NADH-dependent methaemoglobin reductase) reduces it back to Fe(II) functional haemoglobin. Excessive methaemoglobinaemia is a clinical problem and causes cyanosis (bluish discolouration), confusion, and at high levels death by hypoxia.
In the lungs (alveolar pO₂ ≈ 100 mmHg), oxygen binds reversibly to the sixth coordination site of Fe²⁺ in each haem group. The Fe–O₂ bond is bent at approximately 120° (the Fe–O–O angle), with one oxygen atom coordinated to Fe and the other O pointing away — a "side-on" binding geometry stabilised by the H-bond to distal histidine E7. The complete tetramer can carry up to four O₂ molecules:
mathrmHb+4,O2rightleftharpoonsHb(O2)4
The equilibrium can be written stepwise — Hb + O₂ ⇌ Hb(O₂), Hb(O₂) + O₂ ⇌ Hb(O₂)₂, etc. — but with a crucial twist: the binding of one O₂ makes the next O₂ bind more readily. This cooperative binding is a hallmark of haemoglobin and the basis of its physiological efficiency. The mechanism, worked out by Perutz, is structural: when O₂ binds at one haem subunit, it pulls the Fe²⁺ slightly into the porphyrin plane (the deoxy form has the Fe slightly out of plane, "puckered" towards the proximal histidine; the oxy form is in-plane). This movement of the Fe drags the proximal histidine with it, which in turn rotates the F-helix of the globin subunit, which in turn shifts the quaternary structure of the tetramer from the "tense" (T) state — characteristic of deoxyhaemoglobin, with low O₂ affinity — to the "relaxed" (R) state — characteristic of oxyhaemoglobin, with much higher O₂ affinity. The T → R transition propagates through the tetramer so that all four subunits switch between low-affinity and high-affinity states more or less in concert.
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