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Química Inorgânica · Coordination chemistry

Transferência de elétrons e química bioinorgânica

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O que você vai aprender

  • Distinguish inner-sphere from outer-sphere electron transfer and use labile/inert reasoning and ligand-transfer evidence to assign a pathway.
  • Use qualitative Marcus theory, including reorganization energy, the cross relation and the inverted region, to compare electron-transfer rates.
  • Explain O₂ binding, spin state and cooperativity in myoglobin and hemoglobin, and compare them with other O₂ carriers.
  • Relate the structures of cytochromes, iron–sulfur clusters and blue copper proteins to their roles and potentials as electron carriers.
  • Explain how zinc enzymes, nitrogenase and photosystem II use their metal sites, including the electron and photon bookkeeping.

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Two ways to move an electron between metal ions

In an outer-sphere reaction both coordination spheres stay intact: the two complexes meet, and the electron tunnels from one metal to the other through the ligands and solvent. No metal–ligand bond is made or broken in the electron-transfer step. In an inner-sphere reaction one ligand first bridges the two metals, the electron passes through the bridge, and the bridge usually ends up on the newly oxidized metal. Taube's classic example is the reduction of [Co(NH₃)₅Cl]²⁺ by [Cr(H₂O)₆]²⁺ in acid. Cr(II) (high-spin d⁴) is labile, so it loses a water and binds the chloride of the Co(III) complex, which is low-spin d⁶ and inert, so it keeps its chloride. After the electron moves, Cr(III) (d³) is inert and holds on to the chloride, while Co(II) (high-spin d⁷) is labile and releases its ammonia, which is protonated to NH₄⁺. Every chloride ends up in [Cr(H₂O)₅Cl]²⁺, and added free chloride is not incorporated, so the chloride must have been transferred through a bridge. An inner-sphere path needs a ligand with a spare lone pair (Cl⁻, N₃⁻, SCN⁻, carboxylate) and a labile partner that can open a site; coordinated NH₃ has no spare lone pair, and two substitution-inert partners must react by the outer-sphere route. An ambidentate bridge such as thiocyanate can be attacked at its free end (remote attack), so N-bound Co–NCS gives mainly S-bound Cr–SCN first, which later isomerizes to the more stable Cr–NCS.

Marcus theory: why some electron transfers are slow

An electron moves far faster than nuclei, so it can jump only when the bond lengths and the surrounding solvent have already reached a geometry in which reactants and products have the same energy (the Franck–Condon principle). The energy needed to distort the reactants to the product geometry without transferring the electron is the reorganization energy λ: an inner-sphere part from metal–ligand bond-length changes and an outer-sphere part from solvent repolarization. For a reaction with standard free-energy change ΔG°, Marcus theory gives ΔG‡ = (λ + ΔG°)²/(4λ). The barrier falls as the reaction becomes more downhill, vanishes when ΔG° = −λ, and rises again when ΔG° is more negative than −λ (the inverted region). Couples that add an electron to nonbonding t₂g orbitals with little change in bond length, such as [Ru(NH₃)₆]³⁺/²⁺ or [Fe(phen)₃]³⁺/²⁺, have small λ and fast self-exchange; [Co(NH₃)₆]³⁺/²⁺ changes from low-spin d⁶ to high-spin d⁷, populates σ-antibonding e_g orbitals, lengthens every Co–N bond and exchanges extremely slowly. The cross relation k₁₂ = (k₁₁k₂₂K₁₂f)^½, with f ≈ 1, predicts a cross-reaction rate from two self-exchange rates and the equilibrium constant. In proteins, the rate also falls exponentially with donor–acceptor distance, k ∝ e^(−βr) with β ≈ 1.4 Å⁻¹.

Oxygen carriers: myoglobin and hemoglobin

Myoglobin and each hemoglobin subunit hold one heme: Fe bound by the four nitrogens of a porphyrin dianion and, on one face, by the proximal histidine. Deoxy heme is five-coordinate high-spin Fe(II) (d⁶, four unpaired electrons, spin-only μ = √24 ≈ 4.90 μB) and sits slightly out of the porphyrin plane toward the proximal histidine. O₂ binds on the distal face; the iron becomes low spin, moves into the plane, and the bent Fe–O–O unit is diamagnetic overall, described either as low-spin Fe(II)–O₂ or as low-spin Fe(III) coupled to superoxide. The distal histidine hydrogen-bonds to the bound O₂, which favours O₂ over CO. The globin also hides the heme in a hydrophobic pocket so two hemes cannot meet: free Fe(II) hemes in water are oxidized by O₂ to Fe(III) through Fe–O–O–Fe bridged intermediates, and Fe(III) (met) heme does not carry O₂. When the iron moves into the plane it pulls the proximal histidine and its helix, which shifts hemoglobin from the low-affinity T state toward the high-affinity R state. The result is cooperativity: hemoglobin binds O₂ sigmoidally (Hill coefficient n ≈ 2.8, P₅₀ ≈ 26 torr), while myoglobin binds hyperbolically with much higher affinity (P₅₀ ≈ 2.8 torr). Fractional saturation follows Y = pO₂ⁿ/(P₅₀ⁿ + pO₂ⁿ). Other carriers use different metals: hemocyanin binds O₂ between two copper ions as peroxide (colourless Cu(I) becomes blue Cu(II)).

Electron carriers: cytochromes, iron–sulfur and copper

Cytochrome c has a heme whose iron is held by histidine and methionine. With all six positions filled it cannot bind O₂ and only shuttles electrons, cycling between low-spin Fe(III) (d⁵, one unpaired electron) and low-spin Fe(II) (d⁶); both states keep the electron in t₂g orbitals, so λ is small. Cytochrome P450 instead has a cysteine thiolate on one face and an open site on the other, where it binds and activates O₂. Iron–sulfur clusters carry one electron at a time: ferredoxins use the [4Fe–4S]²⁺/¹⁺ pair, whereas high-potential iron proteins (HiPIPs) use [4Fe–4S]³⁺/²⁺ and therefore have much higher potentials. Blue copper proteins such as plastocyanin bind Cu through two histidines, a cysteine and a methionine in a distorted tetrahedral site, between the square-planar geometry Cu(II) prefers and the tetrahedral geometry Cu(I) prefers. This entatic site keeps λ small. Its intense blue colour (ε in the thousands of M⁻¹ cm⁻¹, near 600 nm) comes from cysteine-S → Cu(II) charge transfer, not from a weak d–d band. In the respiratory chain electrons flow toward higher reduction potential, from NADH (E°′ ≈ −0.32 V) to O₂ (E°′ ≈ +0.82 V), and ΔG°′ = −nFΔE°′ for each step.

Metalloenzymes: zinc, nitrogenase and photosystem II

Zinc enzymes use Zn²⁺ as a Lewis acid. Zn²⁺ is d¹⁰: it has no redox chemistry under biological conditions and no ligand-field preference for one geometry, so it binds substrates and water flexibly without side reactions. In carbonic anhydrase Zn(His)₃ lowers the pKa of bound water to about 7, and the zinc hydroxide attacks CO₂; the same nucleophile hydrolyses activated esters such as 4-nitrophenyl acetate. In alcohol dehydrogenase the zinc binds the alcohol oxygen, lowers its pKa so that a proton relay can remove the O–H proton, and the zinc alkoxide gives a hydride to NAD⁺. Because Zn²⁺ is colourless and diamagnetic, chemists often replace it with Co²⁺, which usually keeps activity and shows d–d bands that report on the site. Nitrogenase reduces N₂ at the iron–molybdenum cofactor (MoFe₇S₉C with homocitrate) of the MoFe protein; the Fe protein, a [4Fe–4S] protein that hydrolyses ATP, delivers the electrons one at a time: N₂ + 8 H⁺ + 8 e⁻ + 16 ATP → 2 NH₃ + H₂ + 16 ADP + 16 Pᵢ. Photosystem II oxidizes water at a Mn₄CaO₅ cluster: 2 H₂O → O₂ + 4 H⁺ + 4 e⁻. Each photon removes one electron, advancing the cluster through the S states S₀ → S₄, and O₂ is released on the S₄ → S₀ step. Dark-adapted samples rest in S₁, so with single flashes the first big O₂ burst comes on the third flash and the yield oscillates with period four.

What to carry forward

Decide the electron-transfer pathway from the ligands and lability of both partners, and judge its rate from the reorganization energy and driving force. In biology, the same ideas explain why heme, iron–sulfur and copper carriers are tuned for fast electron transfer, how hemoglobin and myoglobin bind O₂ reversibly, and how zinc, molybdenum–iron and manganese sites do acid–base and multi-electron chemistry. Keep careful count of electrons, oxidation states and unpaired spins at every step.

Problemas de prática (40)

Praticar é grátis com uma conta.

Desenhe o mecanismo 2

Zinc, nitrogenase and photosystem II

  • Alcohol dehydrogenase: a zinc alkoxide gives a hydride to NAD⁺Draw how zinc-bound ethanol is deprotonated, how the zinc ethoxide transfers a hydride to C4 of NAD⁺ to give acetaldehyde and NADH, and how water releases the acetaldehyde from zinc.
  • Carbonic anhydrase hydrolyses 4-nitrophenyl acetateDraw how the zinc-bound hydroxide of carbonic anhydrase adds to 4-nitrophenyl acetate, how the tetrahedral intermediate expels 4-nitrophenolate, and how water and His64 restore the zinc hydroxide.

Preveja o produto 2

Inner- and outer-sphere pathways

  • Put Taube's reaction in orderSequence the events of an inner-sphere electron transfer from bridge formation to product release.

Marcus theory and reorganization

  • More downhill, but slower?Predict which member of a series reacts fastest using the Marcus inverted region.

Classificar 9

Inner- and outer-sphere pathways

  • Which ligand cannot bridge?Recognize that a coordinated ligand needs a spare lone pair to act as an inner-sphere bridge.
  • What makes a reaction outer-sphere?Identify the defining feature of an outer-sphere electron-transfer reaction.

Cytochromes, iron–sulfur and copper carriers

  • What holds the iron of cytochrome c?Identify the axial ligands of cytochrome c and relate them to its role as an electron carrier.
  • A copper oxygen carrierDescribe how hemocyanin binds O₂, with the electron count and oxidation states of its two copper ions.
  • The proximal ligand of cytochrome P450Identify the proximal ligand of cytochrome P450 from its CO-complex spectrum and its O₂-activating role.

Hemoglobin and myoglobin

  • The iron in deoxyhemoglobinDescribe the oxidation state, spin state and coordination of the iron in deoxy myoglobin and hemoglobin.

Zinc, nitrogenase and photosystem II

  • Who does what in nitrogenase?Assign the roles of the Fe protein, the P-cluster and the iron–molybdenum cofactor in nitrogenase.

Marcus theory and reorganization

  • What goes into λ?Identify the two contributions to the Marcus reorganization energy.
  • Which couple reorganizes least?Choose the redox couple with the smallest inner-sphere reorganization energy from its d-electron configurations.

Comparar 3

Hemoglobin and myoglobin

  • Myoglobin versus hemoglobinCompare the O₂-binding curves of myoglobin and hemoglobin and their roles in O₂ delivery.

Cytochromes, iron–sulfur and copper carriers

  • Follow the electrons to O₂Order respiratory-chain carriers by reduction potential to predict the direction of electron flow.

Marcus theory and reorganization

  • Rank self-exchange ratesRank self-exchange rate constants from the change in metal–ligand bond length between oxidation states.

Calcular 9

Cytochromes, iron–sulfur and copper carriers

  • Energy released per electron to O₂Calculate the standard free-energy change for passing one mole of electrons from cytochrome c to O₂ from reduction potentials.
  • Unpaired electrons in oxidized cytochrome cCount the unpaired electrons of the low-spin iron(III) in oxidized cytochrome c.

Hemoglobin and myoglobin

  • Magnetic moment of deoxy hemeCalculate the spin-only magnetic moment of the high-spin iron(II) in deoxyhemoglobin.
  • How much O₂ does hemoglobin unload?Calculate the fraction of hemoglobin sites that release O₂ between lungs and tissue with the Hill equation.

Marcus theory and reorganization

  • How much slower 5 Å further away?Calculate how much a protein electron-transfer rate falls when the donor–acceptor distance increases, from the exponential distance decay.
  • Marcus activation energyCalculate a Marcus activation free energy from the reorganization energy and the driving force.
  • Predict a cross-reaction rateUse the Marcus cross relation to calculate a cross-reaction rate constant from self-exchange rates and an equilibrium constant.

Zinc, nitrogenase and photosystem II

  • Electrons for three N₂Calculate the number of electrons nitrogenase consumes, including obligatory H₂ formation, for a given amount of N₂.
  • Photons for one O₂Calculate the minimum number of photons the two photosystems absorb to release one O₂ by linear electron flow.

Encontre o erro 1

  • Is a chloride enough for an inner-sphere path?Diagnose why a bridging ligand on the oxidant does not make a reaction inner-sphere when the reductant is substitution-inert.

Interprete as evidências 4

Cytochromes, iron–sulfur and copper carriers

  • Why so intensely blue?Assign the intense visible band of a blue copper protein from its intensity and oxidation-state dependence.

Zinc, nitrogenase and photosystem II

  • Oxygen from flashes of lightInterpret the flash-number pattern of O₂ evolution from photosystem II in terms of the S-state cycle.

Inner- and outer-sphere pathways

  • Faster than any ligand can moveUse a comparison of electron-transfer and ligand-substitution rates to assign an outer-sphere mechanism.
  • Where did the chloride go?Interpret the chloride-transfer and labelling results of the Cr(II)/[Co(NH₃)₅Cl]²⁺ reaction as evidence for a bridged intermediate.

Trabalhe de trás para frente 1

  • Which linkage isomer was reduced?Work back from the linkage isomer of the chromium product to the linkage isomer of the cobalt oxidant in a remote-attack reduction.

Planeje uma síntese 1

  • Making a silent metal site visibleChoose a metal substitution that turns a spectroscopically silent zinc site into an informative probe.

Explique por quê 8

Cytochromes, iron–sulfur and copper carriers

  • The strained copper siteExplain how the geometry of a blue copper site makes electron transfer fast.
  • Same cluster, opposite potentialsExplain why a high-potential iron protein and a ferredoxin with the same [4Fe–4S] core have very different reduction potentials.

Hemoglobin and myoglobin

  • How does one heme talk to the others?Explain how a change at one heme iron is transmitted to the other subunits of hemoglobin.
  • How myoglobin tames COExplain how the distal histidine shifts myoglobin's preference toward O₂ relative to CO.
  • Why heme needs its proteinExplain how the globin prevents the irreversible oxidation that free iron(II) hemes undergo with O₂.
  • Why is oxyhemoglobin diamagnetic?Explain why oxyhemoglobin is diamagnetic although both deoxy heme and O₂ are paramagnetic.

Marcus theory and reorganization

  • Why does self-exchange have a barrier?Explain from the Franck–Condon principle why even a thermoneutral self-exchange reaction has an activation barrier.

Inner- and outer-sphere pathways

  • Why does the chloride end up on chromium?Explain the outcome of Taube's reaction from the lability of each metal before and after electron transfer.

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