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 sigma-electrons and an antibonding sigma*-electron in the sulfur-sulfur bridge, (-S therefore 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. The (-S therefore S-)(-) bonded radical anion shows a broad optical absorption band with lambdamax=450 nm, epsilonmax=6000 M(-1) cm(-1), and a half-width of 1.0 eV. It exists in equilibrium with the conjugated 1-(2',3',5'-tri- O-acetylribosyl)uracil-4-yl thiyl radical -S(*), and the corresponding thiolate, -S(-). The rate determining step for the disappearance of the disulfide radical anion appears to be protonation of both the radical anion and the free thiolate by reaction with H(+)aq. Absolute rate constants have been measured for these protonation processes, for the formation of the stable thiouridine product, and for the electron transfer from the disulfide radical anion to molecular oxygen. With the (-S therefore S-)(-) <--> -S(*) + -S(-) equilibrium lying very much on the left-hand side, the reduced disulfide system exhibits predominantly reducing properties whereas any oxidizing property of the conjugated thiyl radical has only little if any chance to materialize. Besides attaching directly to the disulfide bridge, the hydrated electrons react also, with about equal efficiency, with the uracil moiety of the investigated compound. This leads to a structurally totally different and electronically distinguishable species than that with the reduced disulfide bridge. In particular, there is no face-to-face interaction between the two heterocyclic moieties and no increased electron density in the S-S bond. The C-centered radicals resulting from the reduction of the uracil and possibly also generated from the ribosyl moieties initiate further cleavage of the S-S bond and thus contribute to the formation of thiouridine. The overall yield of the latter, as determined from steady-state gamma-radiolysis, indicates a small chain process (G=1.54 micromol/J). Possible mechanisms are discussed.
Wenska
Grazyna
G
Faculty of Chemistry, Adam Mickiewicz University, 60-780 Poznan, Poland. gwenska@amu.edu.pl
Filipiak
Piotr
P
Asmus
Klaus-Dieter
KD
Bobrowski
Krzysztof
K
Koput
Jacek
J
Marciniak
Bronislaw
B
D000838:Anions; D004220:Disulfides / Q000737:chemistry*; D004583:Electrons; D010084:Oxidation-Reduction; D013455:Sulfur / Q000737:chemistry*; D014867:Water
D000838:Anions; D004220:Disulfides; D014867:Water; D013455:Sulfur