Mechanism of Hydroxyl Radical Addition to Imidazole and Subsequent Water Elimination

The addition reaction of the hydroxyl radical to imidazole and subsequent elimination of water to form the 1-dehydroimidazolyl radical is investigated using MP2 and B3LYP methods, including large basis sets and SCI-PCM modeling of solvent effects. It is found that the barrier to addition of the hydroxyl radical at the 5-position is energetically favored over addition to the 2- or 4-positions by 2−3 kcal/mol at the SCI-PCM/MP2/6-311G(2df,p)//MP2/6-31G(d,p) level, whereas the corresponding B3LYP calculations yield a barrier-free addition at the 5-position. The lower barrier and NBO analysis explain the experimentally observed specificity for the 5-hydroxylation of imidazole and histidine, albeit the 2-adduct is about 4 kcal/mol more stable than the 5-adduct. The NBO energetic analysis shows that the exoanomeric effect stabilizes the transition state at the 5-position about 0.3 kcal/mol more than that at the 2-position. Moreover, the π-interaction between the attacking nonbonding spin orbital of the hydroxyl radical and the π-cloud of imidazole is the least for the transition state at the 5-position, favoring the σC5-O bond formation. The 5-hydroxyimidazolyl radical undergoes a slow elimination of water (the added OH group and the hydrogen at the N1 position) to yield the 1-dehydroimidazolyl radical. The base-catalyzed dehydration profile was modeled in two steps at the B3LYP/6-311G(2df,p)//6-31G(d,p) level. The PES for the dehydration reaction seems rather flat. The first step is a barrier-free loss of the proton at N1 induced by the hydroxide ion to yield the 1-dehydro-5-hydroxyimidazolyl radical anion. In the second step, the hydroxide ion is regenerated from the intermediate to yield the final product with a barrier of 2.7 kcal/mol. The calculated hyperfine structures in the presence of the continuum solvent model for the 5-hydroxyimidazolyl and 1-dehydroimidazolyl radicals are in close agreement with the experimental ones recorded in aqueous solution.