Answers to Problem Set 7

PROBLEM SET 7
Oxygen and Transition Complexes

This problem set continues with principles electron transport. It focuses on the structure of oxygen and transition metal complexes.

1. Electrons are passed one at a time through the electron transport system of oxygen. Using your MO diagram for oxygen (Problem Set 6), predict the types of molecules that will form when electrons are added one at a time.

ANS: See p. 174 section 7.12 of The Chemical Basis of Metabolism for the answer.

2. Iron is a transition metal. It has 18 electrons. Two of the valance electrons are in the 4s orbital. The remainder are in the 3 d orbitals. Show the orbital distribution of the valance electrons of Fe and Fe+2.
Hint: The 4s electrons are lost first.

ANS: See Fig. 7.27

3. Explain how Fe+2 can have coordination numbers of either 4 or 6.
ANS: Fe+2 has an empty 4s orbital and three empty 4p orbitals. The energy levels of these atomic orbitals are close enough so they can hybridize to form four sp3 orbitals that can combine with four ligands, each carrying an electron pair, to form four bonding and four antibonding MOs. Since sp3 orbitals for a tetrahedron, the coordination complex will be tetrahedral.
Alternatively, all the electrons in the 3d orbitals can be paired. This leaves two empty 3d orbitals. The 3dx2-y2 and3 dz2 orbitals have the correct symmetry to combine with the 4s and 4p orbitals to form six hybrids with octahedral geometry. These hybrids can combine with six ligands to give a coordination number of 6.

4. Desulforedoxin type proteins, which occur in complex I, and rubredoxin type proteins, which are found in complex II, form a transition complex with either ferrus or ferric ions and four cysteine residues from the proteins.
a. Show what these complexes look like.
b. What do the roman numerals mean?
c. What are the charges on the ferrus and ferric complexes?
d.Explain how oxidation/reduction can occur without disturbing the coodination complex.

ANS:
4a. See CBM Fig 7.29
4b. The roman numerals indicate the oxidation level.
4c. Each cysteine residue contributes a negative charge (total –4). Ferrus ion (FeII) is +2. Therefore, the ferrus complex has a net charge of –2. Ferric ion (FeIII) has a charge of +3. Therefore, the ferric complex has a net charge of –1.
4d. The coordination complex is formed from four 4s4p hybrid orbitals. Fe+2 has 7 valance electrons; Fe+3 has 6. They are located in 3d orbitals:

Since the valance electrons are not involved in making the coordination complex gain or loss of a valance electron does not disrupt the coordination complex.

5. Some of the iron-sulfur proteins in the electron transport system are of the general type Fe2S2. They use an inorganic sulfide ion and four cysteine residues to form a coordination complex. Show what the complex look like.

ANS: See Fig. 7.30 of CBM

6. Cytochrome c is an iron phorphyrin protein. It uses methionine 80, histidine 18 and the porphyrin ring of protophorphyrin IX to form a transition complex. Disregarding the side chains of the porphyrin ring, show what the complex looks like.

ANS: See Fig. 7.38

7. Copper is a transition metal with 11 valance electrons. Show the electron distribution of Cu, Cu+ and Cu++.

ANS: See Fig. 7.28 of CBM

8. Cytochrome oxidase uses His 181 and 224, Met 221, the peptide bond of Glu 218, CyS 216 and 220 to for a Cu2CyS2 type coordinate complex with either Cu+ or Cu++. Show what the complex looks like.

ANS: See section of 7.18 of BM

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