Electron Transfer - Bioinorganic Chemistry | CHEM 332, Study notes of Inorganic Chemistry

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Bioinorganic Chemistry

Electron Transfer Electron Transfer

CHEM-

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Electron Transfer

Definition Definition

Electron transfer:

• (^) is the process by which electrons are transferred from one molecule or atom to another atom or molecule.
• (^) describes the mechanisms involved in the oxidation-reduction process.
• (^) is involved in a large number of biological processes, including respirations and photosynthesis.

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Electron Transfer

Noncomplementary Electron Transfer Noncomplementary Electron Transfer

Electron transfer is called noncomplementary noncomplementary when the number of species being oxidized is different than those of species being reduced. In noncomplementarynoncomplementary electron transfer reactions the ratio of the species being oxidized to the species being reduced is not 1 to 1. Example: [ Pb Cl 6

]

2-

• 2 Cr Cl 2  [Pb Cl 3

]

• • 2 Cr Cl 3 + Cl -

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Electron Transfer

Outer-Sphere Outer-Sphere Electron TransferElectron Transfer

Outer-sphere Outer-sphere electron transfer reactions involve molecules/atoms in the outer coordination sphere in the electron transfer process. In outer-sphereouter-sphere electron transfer reactions the inner sphereinner sphere coordination remains unchanged. Example:

[ Co (NH

3

6

]

3+

+ [ Ru (NH

3

6

]

2+

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Electron Transfer

Electron Transfer Centers Electron Transfer Centers

There are three common types of electron transfer centers in biological systems involving: 1.Cysteine dimer formation. 2.Small organic molecules such as:

• (^) Nicotinamide adenine dinucleotide (NAD)
• (^) Quinones
• (^) Flavin mononucleotide (FMN) 3.Metalloproteins, such as in

0808 Metal Centers in Electron Transfer Reactions

Metal Centers in Biomolecules Metal Centers in Biomolecules

The active role of metals in biomolecules is greatly dependent on:

• (^) The stable oxidation states of the metal
• (^) The ligands available from the amino acids that form the protein chains.
• (^) The geometry of the inner-sphere ligands surrounding the metal atom.
• (^) The ability of substrates and other ligands and cofactor to reach the metal center.

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Electron Transfer

Electron Transferases Electron Transferases

The third type are also called electron transferases , and they have the following characteristics:

• (^) They have associated cofactors acting as electron sources or sinks, i.e. NAD + /NADH.
• (^) There is enough space to allow the cofactor close enough for electron transfer.
• (^) A protein hydrophobic shell close to the cofactor.
• (^) Flexible enough chain to allow conformational changes to facilitate the electron transfer process.

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Electron Transfer

Electron Transferases Electron Transferases

An example of electron transferases are the large and varied family of cytochrome biomolecules. They include:

• (^) Cytochromes, four different ones are known in the electron transfer process in mitochondria.
• (^) Rubredoxin, contain iron-sulfur centers, where the irons are ligated to four cysteine sulfurs.
• (^) Ferredoxin, found in plants and animals an contain [2Fe-2S] cluster centers.