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

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Presentation on theme: "The Molybdenum Cofactor: Moco What early experiments indicated: … but it wasn’t entirely correct."— Presentation transcript:

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

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

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

11 What is the sulfur donor ligand for the family of related Moco structures in molybdenum enzymes? From X-ray diffraction of protein crystals, ~1994

12 Mo The Ligand is called Molybdopterin pterin dithiolene nucleotide

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

14 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

15 Propylene Sulfide - C 3 H 6 S Expected reactivity?? Purpose??Source of sulfur, formally S 0 Related to the Blue Reaction:

16 Sulfur is strange!!!! That’s why the alchemists loved it. They thought that everything, every substance could be made from the “proper” mixture of sulfur, mercury and salt. Sulfur does not behave like its smaller cousin, O. Elemental form: S 8 vs. O=O As ions: S 2- & [S 2 ] 2- O 2- & [O 2 ] 2- & [O 2 ] - polysulfides [S 3 ] 2- & [S 5 ] 2- As ligands: M=S M(S 2 ) M(S 3 ) M(S 4 ) M(S 5 ) disulfide trisulfide tetrasulfide pentasulfide

17 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 Infrared Spectroscopy helps assign sulfur ligand type: M=S has M=S ~ 450-500 cm -1 M(S 2 ) has M-S ~ 500-550 cm -1 M(S 3 ), M(S 4 ) has M-S < 480 cm -1 Let’s try it …

18 [Tp*Mo(X)(S 4 )] — What is X?

19 Mo=S B-H C=N C-H O-H ring compound was determined to be [Tp*Mo(S)(S 4 )] —

20 Mo=S Mo=O Product was determined to be a mixture of both [Tp*Mo(S)(S 4 )] — and [Tp*Mo(O)(S 4 )] —

21 What happens in reactions with Mo complexes? characteristics: Mo(4+): could be oxidized or reduced open (vacant) coordination site Mo loves S + S ? characteristics: Mo(6+): filled coordination sphere Mo loves S Seems OK but is it right? How could we know for sure?????


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