The Molybdenum Cofactor: Moco

What early experiments indicated:

… but it wasn’t entirely correct.

Nit-1 Nit-1 Nit-1 Nit-1

active nitrate reductase

nitrate reductase

sulfite oxidase

xanthine dehydrogenase

aldehyde oxidase

apo NRapo SO apo XDH apo AO

dissociatedMoco

O

Mo OS

SX

O

Mo SS

SX

O

Mo OHS

SX

O

MoS

S

X

S

S

(a) (b) (c) (d) (e)

O

MoS

S

O

S

S

ASP

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

N

SSMo

O O

Mo(6+)O2(L-NS2)

N

SSMo

O

Mo(4+)O(L-NS2)

sol

Mimicking the Catalytic Reactions of Moco

Mo(6+)O2(L-NS2) Mo(4+)O(L-NS2)

PPh3O=PPh3

Me-S(=O)-MeMe-S-Me

DMSO = dimethylsulfoxideDMS = dimethylsulfide

Berg-Holm Model

Mo(6+)O2(L) + 2 H+ Mo(4+)O(L) + H2O

SO32- SO4

2-

2 e- acceptor (like FAD+ )

2 e- reduced (like FADH2

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)

O

Mo+6

OS

S

O

MoOS

S

O

Mo+4

S

S

O

Mo+4

OS

S

SO

OO

OS

O

OO

O

Mo+4

S

SH

H

O

Mo+5

OHS

S

a

- H+, - e -

- H+, - e -

+ SO32-

- SO42-

+ H2O

oxygen

atom

transfer

coupled

proton

electron

transfer

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

Why such a BIG Ligand on Mo?

Mo

S

S O

S

O

S

Mo

S

S

S

O

SX X=O

1st equivalent

Mo

S

S O

S

O

S

Mo

S

S

S

OS

MoSS

S

O

S

Mo SS

O

S

O

S

conproportionation2nd equivalent

•Mo(5+)- Mo(5+) dimer•CN = 6•Terminal vs bridging Mo-O

Step 1:

Step 2:

Typical Mo-oxo Chemistry

BIG Ligand is intended to prevent dimerization

Except… it didn’t!!!Later researchers showed formation of

(L-NS2)Mo

O

O Mo(L-NS2)

O

O

Mo OS

SX

O

Mo SS

SX

O

Mo OHS

SX

O

MoS

S

X

S

S

(a) (b) (c) (d) (e)

O

MoS

S

O

S

S

ASP

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

S

S

O

HN NH

HNNO

O

NH2

MoO NN

NH2

O

HO

P OP O

O

O

OO

OH

From X-ray diffraction ofprotein crystals, ~1994

Mo

The Ligand is calledMolybdopterin

pterin

dithiolene

nucleotide

This week’s reactions:

1. MoO2(detc)2 + excess PPh3 --> RED

2. RED + propylene sulfide --> BLUE

3. Chromatography to separate:

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

Propylene Sulfide - C3H6S

S + "S"

Expected reactivity??

Purpose?? Source of sulfur, formally S0

Related to the Blue Reaction:

Sulfur is strange!!!! That’s why the alchemists loved it. They thought that everything, every substancecould be made from the “proper” mixture of sulfur, mercury and salt.

Sulfur does not behave like its smaller cousin, O.

Elemental form: S8 vs. O=OS

S SS

S

SS

S

As ions: S2- & [S2]2- O2- & [O2]2- & [O2] -

polysulfides [S3]2- & [S5]2-

As ligands: M=S M(S2) M(S3) M(S4) M(S5)M

S S

M

S S

S

M

S S

S S

MoS

SS

S

S

disulfide trisulfide tetrasulfide pentasulfide

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(S2) has M-S ~ 500-550 cm-1 M(S3), M(S4) has M-S < 480 cm-1

Let’s try it …

[Tp*Mo(X)(S4)]—

N

MoS

XN S

NN

N

N

HB

S

S

What is X?

Mo=S

B-H

C=NC-H

O-H ring

compound was determined to be [Tp*Mo(S)(S4)]—

Mo=SMo=O

Product was determined to be

a mixture of both

[Tp*Mo(S)(S4)]— and

[Tp*Mo(O)(S4)]—

What happens in reactions with Mo complexes?

Mo

S

S

S

OS

characteristics:• Mo(4+): could be oxidized or reduced• open (vacant) coordination site• Mo loves S

+ SMo

S

S S

S

O

S

?

characteristics:• Mo(6+):• filled coordination sphere• Mo loves S

Seems OK but is it right?How could we know for sure?????