Presentation on theme: "The Molybdenum Cofactor: Moco"— Presentation transcript:
1The Molybdenum Cofactor: Moco What early experiments indicated:… but it wasn’t entirely correct.
2Now we know that there is not just one Moco, but a family of related Moco structures inmolybdenum enzymes:
3Questions asked of molybdoenzymes and their model compounds: What is the redox potential ( energy of) Moredox reactions?What are the structural details? What is bondorder? (angles, bond distances)How well do models mimic reactions of Moin enzymes? in structure? in reactivity?
4A “second generation” Moco model: the Holm-Berg model
5Mimicking the Catalytic Reactions of Moco Sulfite OxidaseBerg-Holm Model
6What would the Berg-Holm model system suggest? a 2 e- process between Mo(6+) and Mo(4+), and onlythe Mo(6+) and Mo(4+) ox. states are required.BUT, it was known that Mo(5+) plays a role (by EPR)
7A proposed mechanism for Moco Catalysis of Sulfite Oxidation: OAT and CEPT
14Technique: Column Chromatography Review: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-polarfirst, most polar last.Different species may be selectively eluted with increasingthe polarity of solvents, e.g.,CH2Cl2, then acetone, then methanol
15Related to the Blue Reaction: Propylene Sulfide - C3H6SExpected reactivity??Purpose??Source of sulfur, formally S0
16As ligands: M=S M(S2) M(S3) M(S4) M(S5) 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: S vs. O=OAs ions: S & [S2]2- O2- & [O2]2- & [O2] -polysulfides [S3] & [S5]2-As ligands: M=S M(S2) M(S3) M(S4) M(S5)disulfide trisulfide tetrasulfide pentasulfide
17Technique: Infrared Spectroscopy Application to Berg-Holm model and Mo-dimer:detection of Mo=O groupsnumber of absorption related to number Mo=Ofrequency related to Mo oxidation statefrequency also reveals Mo-O-MoInfrared Spectroscopy helps assign sulfur ligand type:M=S has nM=S ~ cm-1M(S2) has nM-S ~ cm-1M(S3), M(S4) has nM-S < 480 cm-1Let’s try it …
19compound was determined to be [Tp*Mo(S)(S4)]— B-HC=NC-HO-Hring
20Product was determined to be a mixture of both[Tp*Mo(S)(S4)]— and[Tp*Mo(O)(S4)]—Mo=SMo=O
21? What happens in reactions with Mo complexes? + S characteristics: Mo(4+): could be oxidized or reducedopen (vacant) coordination siteMo loves Scharacteristics:Mo(6+):filled coordination sphereMo loves SSeems OK but is it right?How could we know for sure?????