Abstract
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, CH3-CO-CH2-COOCH3) at 77 K and subsequent reactions of the anion radical (CH3-CO•−-CH2-COOCH3) in the 77 to ca. 170 K temperature range have been investigated in homogeneous H2O and D2O 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 CH3-C(•)OH-CH2-COOCH3. The ESR spectrum of CH3-C(•)OH-CH2-COOCH3 in H2O shows line components due to proton hyperfine couplings of the methyl and methylene groups. Whereas, the ESR spectrum of CH3-C(•)OH-CH2-COOCH3 in D2O glass shows only the line components due to proton hyperfine couplings of CH3 group. This is expected since the methylene protons in MAA are readily exchangeable in D2O. On stepwise annealing to higher temperatures (ca. 150 to 170 K), CH3-C(•)OH-CH2-COOCH3 undergoes bimolecular H-atom abstraction from MAA to form the more stable radical, CH3-CO-CH•-COOCH3. Theoretical calculations using density functional theory (DFT) support the radical assignments.
Highlights
Ionizing radiation transfers energy to matter by the production of holes, ejected electrons, and excited states [1]
This work leads to the following salient conclusions: (i) In methyl acetoacetate (MAA), the added electron localizes at the carbonyl group: The combination of electron spin resonance (ESR) and density functional theory (DFT) calculations presented in this work clearly shows that in a molecule with a carbonyl and ester functionality separated by a methylene bridge, the site of prehydrated electron attachment is preferentially at the carbonyl group
The spin density distribution of the initial anion radical shows the spin is shared between the two sites with increased localization at the carbonyl occurring on protonation of the carbonyl oxygen
Summary
Ionizing radiation transfers energy to matter by the production of holes, ejected electrons, and excited states [1]. For many molecular systems with functional groups that can capture electrons such as ketones, esters, carboxylic acids, and peptides, it is well known that the anion radicals formed on epre− addition undergo subsequent reactions [8,9,10,11]. The anion radicals formed on electron attachment lead to reactive intermediates that may undergo bimolecular H-atom abstraction reactions. This is shown below for acetone anion radical, pKa = ca. We have investigated prehydrated electron attachment to methyl acetoacetate (MAA) in homogeneous aqueous (H2O or D2O) glasses at 77 K and followed the subsequent reactions of the MAA anion radical in the 77 to ca. The protonated MAA anion radical is found to undergo bimolecular H-atom abstraction from the parent compound (i.e., MAA)
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