Arsenic-based DNA

A second chemical reason (besides hydrolytic instability) why an arsenate-based structure would be unsuitable as a genetic material is that it would be vulnerable to reduction by cellular reductants such as the thiol glutathione (GSH)  

We know that, in vitro, a variety of reducing agents, including glutathione, can reduce arsenic acid (H3ASO4) to arsenous acid. With iodide, for example, the reaction is  

Both polar and radical mechanisms may be envisioned for the process. A polar mechanism might begin with E, a good nucleophile, attacking a positively charged and pentavalent As, a good electrophile, to create an I-As bond  

The actual reduction step would then involve a second iodide attacking the As-bound I to produce molecular iodine, while kicking out an arsenite anion (H2As03 ) as the leaving 

A radical mechanism, on the other hand, is also conceivable. In this case, iodide would begin by transferring a single electron to arsenic acid, producing a tetravalent As intermediate, which could fall apart in a number of ways, one of which is shown below  

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