Lecture 5

Question 1

How is energy y harvested from the sun?

Answer 1

Directly - photosynthesis

indirectly - using photosynthesising organisms as fuel

Question 2

What is energy in electrochemical terms?

Answer 2

a flow of electrons from fuel to oxidant

Question 3

Examples of Fuels and Oxidants

Answer 3

Fuels - fats, sugars, H2

Oxidants - O2, nitrate and H+

Question 4

Three types of proteins used to achieve electron flow

Answer 4

• blue copper
• iron- sulphur
• cytochromes

Question 5

How do donors affect reduction potential?

Answer 5

• Strong donors stabilise high oxidation states, and lower the reduction potential
• Weak donors, pi acceptors and protons all stabilise low oxidation states and raise the reduction potential

Question 6

What other than donors affects reduction potential?

Answer 6

• relative permittivity
• media
• hydrogen bonding interactions

Question 7

What is Marcus Theory?

Answer 7

• explains rate of electron transfer

- the lower the reorganisation energy the faster the reaction

Question 8

What are Blue copper proteins? Give 2 examples

Answer 8

• Small proteins bound to a single Cu atom and are so called due to their intense blue colour when oxidised
• plastocyanins used in the photosynthetic pathway between Photosystem 1 and 2
• Azurin found in bacteria, involved in electron transport between [NO3]- to N2

Question 9

Structure of plastocyanin and azurin

Answer 9

• Plastocyanin, Three closely bound donors (one Cys and 2 His) and a weaker Met donor
• Azurin has an additional weak coordination from the Gly O atom
• in both cases the Cu centre is protected from other molecules by the protien

Question 10

Geometry of Blue Copper Proteins

Answer 10

• Rigid B-barrel structure, with fixed Cu coordination geometry
• Coordination centre suits both Cu (I) and Cu (II) centres so there is rapid electron transfer
• Bond lengths only increase by 5-10 pm on going from Cu (II) to Cu (I)

Question 11

Distinctive Characteristics of type on blue copper Proteins

Answer 11

• intense blue colour in the Cu (II) state at 600 nm
• High but variable reduction potential (0.35 V)
• EPR with small hyperfine coupling to the Cu nucleus in Cu(II)

Question 12

What causes Blue Copper proteins intense colour?

Answer 12

• Due to S(Cys) and Cu(II) LMCT

- high intensity means it can’t be a d-d transition

Question 13

What causes hyperfine coupling?

Answer 13

• 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 since the electron is delocalised onto the Cys S and “spends more time” away from the Cu centre.
• 40% of the time on the S(Cys) leading to a highly covalent Cu- S(Cys) bond.

Question 14

Iron-sulphur proteins

Answer 14

• high spin Fe(III) and Fe(II) centres, tetrahedrally coordinated by sulphur ligands
• classified by how many iron and sulphide atoms they contain

Question 15

Rubredoxins vs Ferredoxins

Answer 15

• Rubredoxins contain one iron centre

- Ferredoxins contain 2,3,4 iron centres

Question 16

What is the biological function of Fe-S proteins?

Answer 16

• essential in electron transfer processes such as photosynthesis and cell respiration
• In nitrogen fixation
• Catalytic sites in hydrogenases

Question 17

[1Fe-0S]

Answer 17

• in some types of bacteria
• contain a high spin Fe coordinated in a distorted tetrahedral fashion to four Cys residues
• Single e- redox
• redox potential is sensitive to the conformation of a protein chain

Question 18

[2Fe-2S]

Answer 18

• mammals, plants and bacteria
• two tetrahedral Fe with bridging S2- ions
• extra iron centre allows a greater range of reduction potentials
• negative R.P. means in their reduced form they are good reducing agents
• e- is localised to one iron centre giving a high spin d6-d5 complex (S= 0.5)
• In the oxidised form the two high spin d5 Fe atoms antiferromagnetically couple to give a diamagnetic complex (S=0)

Question 19

Rieske protein

Answer 19

• important subset of [2Fe-2S]
• 2 Cys and 2 His groups
• carries out electron transfer in the photosynthetic pathway
• Minor structural change causes change in reduction potential to + 0.29 V

Question 20

[4Fe-4S]

Answer 20

• cubic
• Fe and S- on alternate corners
• Fe is often further coordinated by Cys or His
• e- delocalised over all 4 iron centres
• minimal bond length change on reduction
• fast transfer due to low reorganisation energy

Question 21

3 Fe(III).Fe(II) [4Fe-4S]

Answer 21

• Found in High potential protiens
• used in anaerobic electron transfer
• in photosynthetic bacteria

Question 22

[3Fe-4S]

Answer 22

• cube with one missing corner
• spin frustrated when reduced
• E = +0. 1 to -0.4 V
• Large range due to many arrangements

Question 23

Model Systems

Answer 23

• hard to make due to redox and polymerisation reactions

- used to study metalloproteins simply

Question 24

Cytochrome structure

Answer 24

• Haem proteins
• Fe ox/red and low spin
• 6 coordinate, 4 to Haem and 2 to amino acids
• axial bonds are amino acids
• very fast electron transfer

Question 25

Bonding in cytochromes

Answer 25

- T2g non-bonding form a pi-overlap with the pi star anti bonding MO of the porphyrin ring - extends d orbitals, enhancing electron transfer and also reducing the distance the electron has to travel

Question 26

Mitochondrial cytochrome c

Answer 26

- mitochondrial inter membrane space where it supplies electrons to cytochromes c oxidase at the end of the respiratory chain - the axial ligands are His and Met - Met is a neutral, soft donor which stabilises Fe(II) better than Fe(III) giving a high 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 are important in allowing the protein to dock with the cytochrome c oxidase enzyme and transfer its electron