Mechanism of hydroxyl radical addition to imidazole and subsequent water elimination

The addition reaction of the hydroxyl radical to imidazole and subsequent e limination of water to form the 1-dehydroimidazolyl radical is investigated using MP2 and B3LYP methods, including large basis sets and SCI-PCM modeli ng of solvent effects. It is found that the barrier to addition of the hydr oxyl radical at the 5-position is energetically favored over addition to th e 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 barri er-free addition at the 5-position. The lower barrier and NBO analysis expl ain the experimentally observed specificity for the 5-hydroxylation of imid azole and histidine, albeit the 2-adduct is about 4 kcal/mol more stable th an the 5-adduct. The NBO energetic analysis shows that the exoanomeric effe ct stabilizes the transition state at the 5-position about 0.3 kcal/mol mor e than that at the 2-position. Moreover, the pi-interaction between the att acking nonbonding spin orbital of the hydroxyl radical and the pi-cloud of imidazole is the least for the transition state at the 5-position, favoring the sigma(C5-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 dehy dration 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 fir st 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 secon d 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 str uctures in the presence of the continuum solvent model for the 5-hydroxyimi dazolyl and 1-dehydroimidazolyl radicals are in close agreement with the de xperimental ones recorded in aqueous solution.