Formation of a Sandwich-Structure Assisted, Relatively Long-Lived Sulfur-Centered Three-Electron Bonded Radical Anion in the Reduction of a Bis(1-substituted-uracilyl) Disulfide in Aqueous Solution

The one-electron reduction of bis[1-(2′,3′,5′-tri-O-acetylribosyl)uracil-4-yl] disulfide, initiated by hydrated electrons in a radiation chemical study, has been shown to yield 1-(2′,3′,5′-tri-O-acetylribosyl)-4-thiouracil as a stable molecular product. The reduction reaction leads, in the first instance, to a transient, albeit remarkably stable disulfide radical anion. This is characterized by a 2-center-3-electron bond with two bonding σ-electrons and an antibonding σ*-electron in the sulfur−sulfur bridge, (−S∴S−)−. It receives its stability from a sandwich-structure with the two uracilyl moieties facing each other (possibly further assisted by the 2′,3′,5′-tri-O-acetylribosyl substituents). A considerable lengthening of the original disulfide bridge from 2.02 to 2.73 A in the radical anion seems to facilitate the interaction of the heterocycles and leads to a gain in stabilization energy of 24 and 33 kcal/mol (100 and 140 kJ/mol) as evaluated by UMP2/cc-pVTZ and UMP2/cc-pVDZ calculations, respectively....