In 3‐iron ferredoxins, the number of sulfide bridges and cysteinyl ligands, respectively, are:
4, 3
3-iron ferredoxins are a type of iron-sulfur protein that plays a role in electron transfer reactions in biological systems. These proteins contain iron-sulfur clusters within their structure, which are crucial for their function.
3-iron ferredoxins Structure
The most common iron-sulfur cluster found in 3-iron ferredoxins is the $\text{[Fe}_3\text{S}_4\text{]}$ cluster. This cluster contains three iron atoms and four sulfide ions. The sulfide ions are inorganic sulfur atoms, distinct from the sulfur provided by amino acids like cysteine.
Sulfide bridges in 3-iron ferredoxins
In the $\text{[Fe}_3\text{S}_4\text{]}$ cluster, the four sulfide ions bridge the three iron atoms. These bridging sulfide ions are often referred to as sulfide bridges. They form the core structure connecting the metal atoms.
- Number of iron atoms: 3
- Number of sulfide ions ($\text{S}^{2-}$): 4
- These 4 sulfide ions act as bridges between the 3 iron atoms.
- Therefore, there are 4 sulfide bridges.
Cysteinyl ligands in 3-iron ferredoxins
Iron-sulfur clusters like $\text{[Fe}_3\text{S}_4\text{]}$ are typically anchored to the protein chain through ligands provided by amino acid residues. In ferredoxins, these ligands are most commonly the thiolate groups ($\text{S}^{-}$) from cysteine residues.
In the $\text{[Fe}_3\text{S}_4\text{]}$ cluster found in 3-iron ferredoxins, three of the iron atoms are usually coordinated by one cysteine thiolate ligand each from the protein. The fourth ligand to each iron is typically a bridging sulfide ion from the cluster itself.
- The cluster is attached to the protein via cysteine residues.
- Each of the three iron atoms that is externally coordinated is typically ligated by a cysteine.
- Therefore, there are usually 3 cysteinyl ligands coordinating the $\text{[Fe}_3\text{S}_4\text{]}$ cluster to the protein.
Based on the structure of the common $\text{[Fe}_3\text{S}_4\text{]}$ cluster in 3-iron ferredoxins and how it's typically attached to the protein:
- The number of sulfide bridges is 4.
- The number of cysteinyl ligands is 3.
Important Questions from Electron Transfer Reactions
Consider the following statements for the self‐exchange electron transfer reaction in [Cr(H2O)6]2+/3+
a. σ* orbitals are only involved in electron transfer
b. It involves large inner‐sphere reorganization energy
c. It involves no change in M–L bond lengths
d. Rate of self‐exchange electron transfer is fast
The correct statements are
The second order rate constants for the outer sphere self- exchange electron transfer reactions for [Ru(NH3)6]2+/[Ru(NH3)6]3+ and [Co(NH3)6]2+/[Co(NH3)6]3+ are 9.2 × 102 M-1 sec-1 and ≤ 10-9 M-1 sec-1, respectively.
The correct rationale for the above data is
In the following electron transfer reactions, the one in which the bridging ligand comes from the reductant is
Of the following assertions regarding the mechanism of electron transfer,
A. An outer-sphere mechanism involves electron transfer from the reductant to the oxidant, with the coordination shells or spheres of each staying intact.
B. In the inner-sphere mechanism, the reductant and oxidant share a ligand in their inner or primary coordination sphere which assists in electrons being transferred.
C. In the inner-sphere mechanism, an oxidant possesses at least one ligand capable of binding simultaneously to two metal ions.
D. In the inner-sphere mechanism, ligands of reductant are substitutionally inert.
The correct statements are
- The water exchange rates for the complex ions follow the order
