Mononuclear Mo and W enzymes require a unique ligand known as molybdopterin (MPT). This ligand binds the metal through a dithiolene chelate, and the dithiolene bridges a reduced pyranopterin group. Pyran scission and formation has 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 pterin or quinoxaline, and a hydroxyalkyl poised to form a pyran ring. hile the pterin-dithiolene model complex exhibits a low energy, eversible pyran cyclization, here we report that pyran cyclization does not spontaneously occur in the quinoxalyldithiolene 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-dithiolene complex 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 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 local protein environment in the second coordination sphere of Moco could control pyran cyclization. The results also demonstrate that the remarkable chemical reactivity of the pterin dithiolene ligand is subtlely distinct and not reproduced by the simpler quinoxaline analog that is often used to replace pterin in synthetic Moco models.
We describe a novel experiment for high school students that uses gas chromatography (GC) equipment to demonstrate the principles of chemical quantitation in the context of fragrant compounds. A highlight of the work is the use of inexpensive equipment and tools accessible to the usual high school laboratory. The students use control samples, learn about functional groups and chemical separations, and are introduced to a commonly used chemistry instrument. We describe the protocols in detail and analyze student survey results to highlight the advantages of this experiment as well as possible routes for further improvement.
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