One-Electron Reduction of 8-Bromo-2-aminoadenosine in the Aqueous Phase:  Radiation Chemical and DFT Studies of the Mechanism

• Published in: The journal of physical chemistry. B 2008-04, Vol.112 (16), p.5209-5217
• Main Authors: Kaloudis, Panagiotis, D'Angelantonio, Mila, Guerra, Maurizio, Gimisis, Thanasis, Mulazzani, Quinto G, Chatgilialoglu, Chryssostomos
• Format: Article
• Language: English
• Subjects: Acids - chemistry; Adenosine - analogs & derivatives; Adenosine - chemistry; Alkalies - chemistry; Electrons; Hydrogen-Ion Concentration; Ions - chemistry; Isotopes; Methylation; Models, Molecular; Molecular Structure; Oxidation-Reduction; Solutions; Stereoisomerism; Water - chemistry
• ISSN: 1520-6106; 1520-5207
• DOI: 10.1021/jp800480f

Two tautomeric forms of one-electron oxidized 2-aminoadenosine (2AA) have been produced by reactions of hydrated electrons (eaq -) with 8-bromo-2-aminoadenosine (8-Br-2AA) at natural pH, whereas only one tautomer is formed by oxidation of 2AA. Tailored experiments by pulse radiolysis and time-dependent DFT (TD-B3LYP/6-311G**//B1B95/6-31+G**) calculations allowed the definition of the reaction mechanism in some detail. The electron adducts of 8-Br-2AA protonated at C8 eject Br- and produce the two short-lived tautomers (8 and 9). The first observable species decays by first-order kinetics to produce the second intermediate, which is also obtained by oxidation of 2AA by SO4 •-. The rate of tautomerization (k taut = 4.5 × 104 s-1) is strongly accelerated by phosphate and is retarded in D2O (kinetic isotope effect 7). B1B95/6-31+G** calculations showed that the tautomerization is a water-assisted process. In acidic or basic solutions, the “instantaneous” formation of one-electron oxidized 2AA or its deprotonated forms has been produced by reactions of eaq - with 8-Br-2AA. γ-Radiolysis of 8-Br-2AA in aqueous solutions followed by product studies led to the formation of 2AA as a single product.