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Bioinorganic, take 2 > Section 4 - Biological Electron Transfer > Flashcards

Section 4 - Biological Electron Transfer Flashcards

1
Q

Where does the energy of life stem from? How is it harvested?

A
  • the sun

- directly via photosynthesis or indirectly using photosynthesising organisms as fuel

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2
Q

How is energy defined in electrochemistry?

A

a flow of electrons from fuel to oxidant

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3
Q

Give examples of fuels and oxidants

A

Fuels: Fats, sugars, hydrogen
oxidants: oxygen, nitrates, H+

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4
Q

What are the threee types of protein use for electron transfer?

A
  • Blue Copper
  • Iron-Sulphur
  • Cytochromes
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5
Q

How can the reduction potential of a redox couple be tuned?

A

by altering the ligands coordinated to a metal centre

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6
Q

What are the reduction potentials of:
[Fe(OH2)6]3+
[Fe(CN)6]3-
[Fe(bpy)3]3+

A

1) 0.77 V
2) 0.36 V
3) 1.03 V

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7
Q

How can you lower reduction potential?

A

Use strong donors to stabilise a high oxidation state

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8
Q

What increases reduction potential?

A

Weak donors, pi acceptors and protons which stabilise low oxidation states

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9
Q

What affects reduction potential other than ligands?

A
  • relative permativity
  • media
  • hydrogen bonding interactions
  • neighbouring charges
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10
Q

How is the rate of electron transfer explained?

A
  • Marcus Theory
  • Consideres organisation energy
  • low reorganisation energy means a faster rate
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11
Q

Describe type 1 blue copper proteins

A
  • Small proteins that bind to a single Cu atom

- give intense blue colour in the oxidised state

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12
Q

Give examples of type 1 blue copper proteins

A
  • Plant chloroplastoc plastocynins, transport electrons from Photosystem 1 to Photosystem 2 in photosynthesis
  • Azurin which is found in bacteria and helps convert [NO3]- to N2
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13
Q

Describe the structure of spinach plastocyanin

A
  • Three closely bound donors (2 His and 1 Cys)

- one weaker Met donor

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14
Q

Describe the structure of azurin

A
  • similar to a spinach plastocyanin

- additional weak coordination from a Gly O atoms

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15
Q

How is the Cu centre protected in azurin and plastocyanin?

A
  • Cu centre is protected from water by the protien
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16
Q

Describe the secondary structure of a Type 1 blue copper protien

A
  • Beta barrel structure holds the Cu coordination sphere in a very rigid geometry
  • coordination sphere suits both Cu (I) and Cu (II) to facilitate rapid transfer
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17
Q

How does bond length increase from Cu(II) to Cu (I) (generally)?

A

5-10 pm

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18
Q

How does bond length change from Cu(II) to Cu (I) in plastocyanin?

A

Cu-N-His37: 2.04-2.12
Cu-S-Cys84: 2.13-2.11
Cu-N-His87: 2.10-2.25
Cu-S-Met92: 2.9-2.9

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19
Q

How does bond length change from Cu(II) to Cu (I) in azurin?

A 
Cu-N-His46: 2.08-2.13
Cu-S-Cys112: 2.15-2.26
Cu-N-His117: 2.00-2.15
Cu-S-Met121: 3.11-3.23
Cu-O-Gly45: 3.13-3.22
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20
Q

What is the distinctive feature of a Type 1 Blue Cu protein in UV-Vis?
What causes it?

A
  • intense peak at 600 nm in the blue oxidised state

- caused by S(Cys) to Cu (II) LMCT

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21
Q

What is the distinctive feature of a Type 1 Blue Cu protein in EPR?
What causes it?

A
  • Simple Cu(II) complexes have large EPR hyperfine coupling to the Cu (I = 3/2 for 65Cu and 67Cu).
  • Blue copper proteins have smaller coupling
  • The electron is delocalised onto the Cys S and “spends more time” away from the Cu centre. Calculated to be 40% of the time on the S(Cys) leading to a highly covalent Cu- S(Cys) bond.
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22
Q

What is the reduction potential of a Type 1 Blue Copper Protein?

A
  • High but variable

- 350 mV compared to less than 100 mV for a typical Cu Complex

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23
Q

Describe the general structure of an Iron Sulpher Protein

A
  • High spin Fe(III) or Fe(II) centres tetrahedrally coordinated by sulphur
  • either S2- or S(Cys)-
24
Q

How are Iron-Sulpher proteins classified?

A
  • according to the number of iron and sulphur atoms they contain
25
Q

What are the two main types of Iron Sulphur Proteins?

A
  • Rubredoxins (one centre)

- Ferredoxins (di, tri or tetra iron centres)

26
Q

Where are Fe-S proteins used in biology?

A
  • essential in photosynthesis and cell respiration
  • used in nitrogen fixation
  • catalytic sites in hydrogenases
27
Q

Describe/ Draw the structure of Rubredoxin

A

High spin Fe coordinated to 4 Cys residues in a distorted tetrahedron

28
Q

Where is Rubredoxin used?

A

In some types of bacteria

29
Q

What is the reduction potential of a Rubredoxin centre?

A
  • 0.05 V to -0.05 V

- sensitive to the conformation of the protein chain

30
Q

What are the bond lengths for Fe(III) and Fe (II) in Rubredoxin?

A

Fe(III) - 2.24-2.33

Fe(II) - 2.3-2.38

31
Q

Where are [2Fe-2S] Ferredoxins found?

A

mammals, plants and bacteria

32
Q

Describe the structure of [2Fe-2S] Ferredoxins

A
  • Two tetrahedral Fe centres, bridged by 2 S2- ions

- each bonded to two S(Cys)-

33
Q

[2Fe-2S] Ferredoxins and single electron transfers

A
  • Fe(III).Fe(III) + e-
    goes to Fe(III).Fe(II)
  • Eo = -0.27 to -0.42 V
  • change from S=0 to S=0.5
  • extra centre allows a range of reduction potentials
  • Negative reduction potential means that in their reduced form they are good reducing agents
34
Q

Why do [2Fe-2S] Ferredoxins change spin state when there is a single electron transfer?

A

In the oxidised form the two high spin d5 Fe atoms couple antiferromagnetically to give a diamagnetic complex (S = 0)

The added electron is localised on one Fe atom in the mixed valence state to give a high spin d6–d5 complex (S = 1⁄2).

35
Q

Describe the structure of the [2Fe-2S] Rieske protein

A
  • Two tetrahedral Fe centres, bridged by 2 S2- ions
  • One Fe bonded to two S(Cys)
  • One Fe bonded to two N(His) groups
36
Q

How does the change in structure between the 2Fe-2S] Ferredoxin and [2Fe-2S] Rieske protein affect the Rieske proteins properties?

A
  • Stabilisation of Fe(II)

- Reduction potential raised to 0.29 V

37
Q

Describe the [4Fe-4S] Ferridoxin structure

A
  • Cubic with Fe and S2- in alternate corners

- Fe further coordinated by Cys or sometimes His residues

38
Q

Describe the single electron transfer of [4Fe-4S] Ferridoxin

A
  • 2Fe(III).2Fe(II) + e- goes to Fe(III).3Fe(II)
  • S=0 to S= 0.5
  • Eo = -0.20 to -0.45 V
39
Q

Why is [4Fe-4S] Ferridoxin electron transfer fast?

A
  • The electrons are delocalised over all four Fe centres.
  • Good electron delocalisation results in minimal bond length changes
  • decreased the organization energy and fast electron transfer.
40
Q

What is found in High Potential Proteins? (HiPIP)

A

3Fe(III).Fe(II) highly oxidised form of [4Fe-4S] Ferridoxin

41
Q

What are HiPIPs used for?

A

Anaerobic electron transport in photosynthetic bacteria

42
Q

What is the single electron transfer reduction potential in HiPIPs?

A
  • 3Fe(III).Fe(II) + e- to 2Fe(III).2Fe(II)
  • S=0.5 to S=0
  • Eo = +0.35 V
43
Q

What Ferridoxins are not know biologically?

A
  • 4Fe(II) clusters

- a single ferredoxin can access all three known oxidation states.

44
Q

Describe the structure of a [3Fe-4S] Ferredoxins

A
  • Cubic with one corner missing
45
Q

Describe the single electron transfer in [3Fe-4S] Ferredoxins

A
  • 3Fe(III) + e- to 2Fe(III).Fe(II)
  • S=0.5 to S=2
  • Eo = +0.1 to -0.4 V
46
Q

Why are model systems used?

A
  • as the study of whole metalloprotiens is difficult

- smaller iron sulphur compounds often used to understand structural, magnetic and electronic properties

47
Q

What are the challenges of model systems?

A
  • the tendancy of iron thiolate systems to undergo redox or polymerisation reactions
  • 2Fe(3+) +2RS(-) to 2Fe(2+) + 2RSSR
  • n Fe(2+) + 2n RS(-) to [Fe(SR)2]n
48
Q

How can model [4F-4S]2+ units be prepared?

A

4 FeCl3 + 12 RS-Na+ goes to 4 Fe(SR)3 + 4NaOMe + 4NaHS goes to Na2[Fe4S4(SR)4] + RSSR + 6NaSR + 4 MeOH

49
Q

What are the properties of model [4F-4S]2+

A
  • Formally 2 Fe(II) and 2 Fe(III) centres

- Spectroscopically all Fe are equal due to the delocalisation of electrons within the cage

50
Q

What are cytochromes?

A
  • Haem proteins which can acts as one electron transfer centres
51
Q

Describe the electron transfer in a cytochrome

A
  • Typically between Fe(II) and Fe (III) forms
  • Potential in the range -0.3 to 0.4 V
  • little change in ligand conformation on reduction/oxidation
  • fast et process
52
Q

Describe the structure of a cytochrome

A
  • Six coordinate
  • 2 stable axial bonds to amino acid donors
  • Fe is normally low spin
53
Q

How does orbital overlap affect the rate of electron transfer in cytochromes?

A
  • The t2g non-bonding orbitals form a π-overlap with the π* antibonding MO of the ring system.
  • extends the d-orbitals out to the edge of the porphyrin ring increasing ET
  • The extension of the d-orbitals also means the distance over which an electron must transfer between redox centres is reduced.
54
Q

How do the main types of cytochromes differ?

A
  • types of peripheral groups on the porphyrin rings

- mode of attachment of the porphyrin to the protein.

55
Q

Describe an example of a cytochrome

A
  • mitochondrial cytochrome c which is found in the mitochondrial intramembrane space where it supplies electrons to cytochrome c oxidase at the end of the respiratory chain.
  • His and Met are the axial ligands
  • Fe (II) is stabilised by the soft Met group giving a reduction potential of 0.26 V
  • The exposed edge of the porphyrin ring is the likely site for electrons to add or leave
  • electrostatic interactions allow the protein to “dock” with the cytochrome c oxidase enzyme and transfer its electron.

Bioinorganic, take 2 (5 decks)

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