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molybdenum
cofactor modeling
--molybdenum
tris-dithiolenes----
. 
---metal pteridine chemistry------ruthenium interactions with DNA--
Molybdenum cofactor modeling
1. Pterins do redox.
Doing redox reactions is characteristic pterin reactivityand hence the
role that most often appears in biochemistry. Surprisingly,
all crystal
structures
of Mo and W enzymes exhibited molybdopterin within the molybdenum
cofactor only in one form, the three ring, pyranopterin form. Therefore,
despite
the
rich
reactivity
possible involving the pterin portion of molybdopterin, no evidence
exists in support of an active redox role for the pterin.
Figure 1 shows
hypothetical redox reactions and intramolecular rearrangements
possible for the pyranopterin within molybdopterin.
Previously, we have investigated the redox chemistry of the pyranopterin system under a variety of oxidative and reductive conditions since little was known about pyranopterin redox reactivity. Redox titrations showed that the pyranopterin system reacts as a dihydropterin, losing 2 electrons in oxidation reactions to produce neopterin. However, pyranopterin does not undergo further reduction to a tetrahydropterin under protic conditions. This suggests that a ring-opened form is not long lived. Note that reduction of neopterin produces either a 7,8-dihydropterin (2 e-) or a tetrahydropterin (4 e-). Therefore, the pyran ring opening is irreversible under these solution conditions. It is important to note that a ring-opened dihydropterin form of molybdopterin was observed in the crystal structure of E. coli nitrate reductase. (Bertero et al, Nature Struct. Biol 2003 10, 681.)
2. The Approach: making Pterin-dithiolene complexes
Substitution of the pyranopterin system on a dithiolene chelate may significantly alter the the stability of ring-opened species. Recent theoretical calculations (DFT) reported by McNamara, Joule Hillier and Garner (Chem. Comm. 2005, 177) on various ring-opened forms of a pyranopterin-dithiolene on oxo-Mo(6+) and Mo(4+) complexes illustrate how the total energy varies with dihydropterin tautomer and protonation state.
We have made a family of oxo- and sulfido-Mo(4+) pterin-dithiolene complexes with the goal of studying the resulting redox chemistry of the chelated pterin-dithiolene and its effect on the electronic environment of the molybdenum. One example of our synthetic approach is given in Figure 2. The pterin-dithiolene chelate is formed from the established reaction of metal polysulfides with suitable alkynes. Our preparations specifically combine a molybdenum tetrasulfide and a pterin-substituted alkyne:
