1. The Molybdenum Cofactor: Moco What early experiments indicated: … but it wasn’t entirely correct.

2. Now we know that there is not just one Moco, but a family of related Moco structures in molybdenum enzymes:

3. Questions asked of molybdoenzymes and their model compounds: • What is the redox potential ( energy of) Mo • redox reactions? • What are the structural details? What is bond • order? (angles, bond distances) • How well do models mimic reactions of Mo • in enzymes? in structure? in reactivity?

4. A “second generation” Moco model: the Holm-Berg model

5. Sulfite Oxidase Berg-Holm Model Mimicking the Catalytic Reactions of Moco

6. What would the Berg-Holm model system suggest? • a 2 e- process between Mo(6+) and Mo(4+), and only • the Mo(6+) and Mo(4+) ox. states are required. • BUT, it was known that Mo(5+) plays a role (by EPR)

7. A proposed mechanism for Moco Catalysis of Sulfite Oxidation: OAT and CEPT

8. Why such a BIG Ligand on Mo?

9. Step 1: Step 2: • Mo(5+)- Mo(5+) dimer • CN = 6 • Terminal vs bridging Mo-O

10. Except… it didn’t!!! Later researchers showed formation of BIG Ligand is intended to prevent dimerization

11. Technique: Infrared Spectroscopy • Application to Berg-Holm model and Mo-dimer: • detection of Mo=O groups • number of absorption related to number Mo=O • frequency related to Mo oxidation state • frequency also reveals Mo-O-Mo

12. Technique: Column Chromatography • Review: • chromatographic separations are based on: • dipolar interaction of molecules with solid support (SiO2) • partitioning of molecule between support and solvent • In practice: • Silica gel column chromatography elutes most non-polar • first, most polar last. • Different species may be selectively eluted with increasing • the polarity of solvents, e.g., • CH2Cl2, then acetone, then methanol

13. This week’s reactions: MoO2(detc)2 + excess PPh3 --> RED RED + propylene sulfide --> BLUE Chromatography to separate:

14. What is the sulfur donor ligand for the family of related Moco structures in molybdenum enzymes?

15. dithiolene pterin The Ligand is called Molybdopterin nucleotide Mo