Abstract
Here we highlight past work on metal-dithiolene interactions and how the unique electronic structure of the metal-dithiolene unit contributes to both the oxidative and reductive half reactions in pyranopterin molybdenum and tungsten enzymes. The metallodithiolene electronic structures detailed here were interrogated using multiple ground and excited state spectroscopic probes on the enzymes and their small molecule analogs. The spectroscopic results have been interpreted in the context of bonding and spectroscopic calculations, and the pseudo-Jahn-Teller effect. The dithiolene is a unique ligand with respect to its redox active nature, electronic synergy with the pyranopterin component of the molybdenum cofactor, and the ability to undergo chelate ring distortions that control covalency, reduction potential, and reactivity in pyranopterin molybdenum and tungsten enzymes.
Highlights
It is well-established that all known molybdenum-containing enzymes [1,2,3], with the sole exception of nitrogenase, contain a common pyranopterin dithiolene (PDT) (Figure 1) organic cofactor (originally called molybdopterin (MPT)), which coordinates to the Mo center of the enzymes through the sulfur atoms of the dithiolene fragment
A common structural feature of the large group of pyranopterin Mo enzymes that catalyze a wide range of oxidation/reduction reactions in carbon, sulfur, and nitrogen metabolism is coordination by the sulfur atoms of one unique dithiolene groups derived from the side chain of a novel substituted pterin (PDT, Figure 1)
Greater understanding of how geometric and electronic structure control reactivity, and define function in Mo and W enzymes, will require linking the concepts that have been developed for metallodithiolenes to the emerging results from studies of well-characterized compounds that mimic the pterin component of PDT (Section 3)
Summary
It is well-established that all known molybdenum-containing enzymes [1,2,3], with the sole exception of nitrogenase, contain a common pyranopterin dithiolene (PDT) (Figure 1) organic cofactor (originally called molybdopterin (MPT)), which coordinates to the Mo center of the enzymes through the sulfur atoms of the dithiolene fragment. The radical anion form with five π-electrons is usually formImportantly, is both a σ-donor and π-donor that dithiolene usually formscan strong covalent an covalent oxidized non-innocent dithiolene ligands modulate thebonds naturewith ofthethe found inthese molecules chelated by multiple ligands, where extended delocalization of π-electrons in the stabilization the metal–ligand bonds.toThe oxidized transition metal ion,and as mixed-valency is observed in the active sites ofof most pyranopterin. W enzymes encountered less frequently high-valent transition ions coordinated by reduced forms dithione form of the much ligand possesses only four π-electrons and can be described two resonance. Dithione-containing metal complexes are dithione form of thesuch ligand possesses only four π-electrons and can low-valent be described by two resonance encountered much less frequently high-valent coordinated structures Dithione-containing low-valent metal complexes are encountered much less frequently than high-valent transition metal ions coordinated by reduced forms of dithiolene ligands. Metal–ligand covalency, reduction potentials, and reactivity in pyranopterin Mo and W enzymes
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