Abstract
Among the chalcogens, selenium is the key element for catalyzed H2O2 reduction. In organic synthesis, catalytic amounts of organo mono- and di-selenides are largely used in different classes of oxidations, in which H2O2 alone is poorly efficient. Biological hydroperoxide metabolism is dominated by peroxidases and thioredoxin reductases, which balance hydroperoxide challenge and contribute to redox regulation. When their selenocysteine is replaced by cysteine, the cellular antioxidant defense system is impaired. Finally, classes of organoselenides have been synthesized with the aim of mimicking the biological strategy of glutathione peroxidases, but their therapeutic application has so far been limited. Moreover, their therapeutic use may be doubted, because H2O2 is not only toxic but also serves as an important messenger. Therefore, over-optimization of H2O2 reduction may lead to unexpected disturbances of metabolic regulation. Common to all these systems is the nucleophilic attack of selenium to one oxygen of the peroxide bond promoting its disruption. In this contribution, we revisit selected examples from chemistry and biology, and, by using results from accurate quantum mechanical modelling, we provide an accurate unified picture of selenium’s capacity of reducing hydroperoxides. There is clear evidence that the selenoenzymes remain superior in terms of catalytic efficiency.
Highlights
Among the chalcogens, selenium is the key element for catalyzed H2 O2 reduction
We revisit selected examples from chemistry and biology, and, by using results from accurate quantum mechanical modelling, we provide an accurate unified picture of selenium’s capacity of reducing hydroperoxides
Selenium is in the core of enzymes deputed to consuming excess of hydroperoxides, which are harmful for the cell, or to exploiting the chemical reaction of hydroperoxide reduction for signalling purpose
Summary
The success of H2 O2 as oxidizing agent has been increasing. Hydrogen peroxide is the ideal oxidant because it is powerful and eco-friendly. When the selenium is protected by cradle-shaped molecular structure, selenenic acids could be synthetized [14,15] As alternative, both H2 O2 molecules can attack the same Se atom forming a hydroxy perhydroxy species (6) (Scheme 4). Using an excess of H2 O2 , the benzene perseleninic acid (8, Scheme 6), which is a powerful oxidant, forms This product is the real oxidizing agent in organic reactions, when H2 O2 is used in presence of a catalytic amount of diphenyl diselenide (Scheme 1). The mechanism of oxidation of an organoselenide and more in general of an organochalcogenide by H2 O2 consists in a nucleophilic attack of the chalcogen (S, Se, Te) to one oxygen of the peroxide with concomitant transfer of its proton to the second oxygen, O-O bond breaking and formation of a water molecule. In the following, (i) the reactivity will be quantitatively described and interpreted using electronic energies and (ii) the energy barriers are computed taking as reference the free reactants, unless differently specified
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