Document Type
Article
Version
Final Published Version
Publication Title
Inorganic Chemistry
Volume
58
Publication Date
2019
Abstract
Mononuclear Mo and W enzymes require a unique ligand known as molybdopterin (MPT). This ligand binds the metal through a dithiolene chelate, and the dithiolenebridges a reduced pyranopterin group. Pyran scission and formation have been proposed as a reaction of the MPT ligand that may occur within the enzymes to adjust reactivity at the Mo atom. We address this issue by investigating oxo−Mo(IV)model complexes containing dithiolenes substituted by pterinor quinoxaline and a hydroxyalkyl poised to form a pyran ring.While the pterin−dithiolene model complex exhibits a low energy, reversible pyran cyclization, here we report that pyran cyclization does not spontaneously occur in the quinoxalyl−dithiolene model. However, protonating the quinoxalyl−dithiolene model induces pyran cyclization forming an unstable, pyrano-quinoxalyl−dithiolene complex which subsequently dehydrates and rearranges to a pyrrolo-quinoxlyl−dithiolenecomplex that was previously characterized. The protonated pyrano-quinoxalyl−dithiolene complex was characterized by absorption spectroscopy and cyclic voltammetry, and these results suggest pyran cyclization leads to a significant change in the Mo electronic structure exhibited as a strong intraligand charge transfer (ILCT) transition and 370 mV positive shift of the Mo(V/IV) reduction potential. The influence of protonation on quinoxaline reactivity supports the hypothesis that the loca lprotein environment in the second coordination sphere of molybdenum cofactor (Moco) could control pyran cyclization. The results also demonstrate that the remarkable chemical reactivity of the pterin−dithiolene ligand is subtly distinct and not reproduced by the simpler quinoxaline analog that is often used to replace pterin in synthetic Moco models.
Citation
Gisewhite, Douglas R., Alexandra L. Nagelski, Daniel C. Cummins, Glenn P. A. Yap, and Sharon J. Nieter Burgmayer. 2019. “Modeling Pyran Formation in the Molybdenum Cofactor: Protonation of Quinoxalyl–Dithiolene Promoting Pyran Cyclization.” Inorganic Chemistry 58 (8): 5134-5144. 10.1021/acs.inorgchem.9b0019.
DOI
10.1021/acs.inorgchem.9b0019
