Presolvated Electron Reactions with Methyl Acetoacetate: Electron Localization, Proton-Deuteron Exchange, and H-Atom Abstraction

Radiation-produced electrons initiate various reaction processes that are important to radiation damage to biomolecules. In this work, the site of attachment of the prehydrated electrons with methyl acetoacetate (MAA, CH(3)-CO-CH(2)-COOCH(3)) at 77 K and subsequent reactions of the anion radical (CH(3)-CO•(−)-CH(2)-COOCH(3)) in the 77 to ca. 170 K temperature range have been investigated in homogeneous H(2)O and D(2)O aqueous glasses by electron spin resonance (ESR) spectroscopy. At 77 K, the prehydrated electron attaches to MAA forming the anion radical in which the electron is delocalized over the two carbonyl groups. This species readily protonates to produce the protonated electron adduct radical CH(3)-C(•)OH-CH(2)-COOCH(3.) The ESR spectrum of CH(3)-C(•)OH-CH(2)-COOCH(3) in H(2)O shows line components due to proton hyperfine couplings of the methyl and methylene groups. Whereas, the ESR spectrum of CH(3)-C(•)OH-CH(2)-COOCH(3) in D(2)O glass shows only the line components due to proton hyperfine couplings of CH(3) group. This is expected since the methylene protons in MAA are readily exchangeable in D(2)O. On stepwise annealing to higher temperatures (ca. 150 to 170 K), CH(3)-C(•)OH-CH(2)-COOCH(3 )undergoes bimolecular H-atom abstraction from MAA to form the more stable radical, CH(3)-CO-CH•-COOCH(3). Theoretical calculations using density functional theory (DFT) support the radical assignments.