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