The Molybdenum Cofactor: Moco

The Molybdenum Cofactor: MocoWhat early experiments indicated: but it wasnt 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

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+) dimerCN = 6Terminal vs bridging Mo-OStep 1: Step 2:

BIG Ligand is intended to prevent dimerization

Technique: Infrared SpectroscopyApplication 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 ChromatographyReview: chromatographic separations are based on: dipolar interaction of molecules with solid support (SiO2) partitioning of molecule between support and solventIn 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

This weeks reactions:MoO2(detc)2 + excess PPh3 --> RED

RED + propylene sulfide --> BLUE

Chromatography to separate:

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

Monucleotide

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