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

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

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?

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

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

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)

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

Why such a BIG Ligand on Mo?

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

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

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

Technique: Column Chromatography Review: chromatographic separations are based on: dipolar interaction of molecules with solid support (SiO 2 ) 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., CH 2 Cl 2, then acetone, then methanol

This week’s reactions: 1.MoO 2 (detc) 2 + excess PPh 3 --> RED 2.RED + propylene sulfide --> BLUE 3.Chromatography to separate:

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

Mo The Ligand is called Molybdopterin pterin dithiolene nucleotide