Ab initioSCF MO calculations on the reactions of hydroxyl radical with imidazole and monoprotonated imidazole

Abstract

The addition reactions of hydroxyl radical with imidazole and its protonated form to yield radical adducts have been investigated by ab initio SCF MO methods using s~o -3 G and 4-31G basis sets. Analogous radical species are of importance in radiation damage to biological systems. Of the possible radical products, the calculations indicate that the allylic species are generally favored energetically over the nonallylic forms. On an energetic basis, the results show that the allylic adducts formed by addition at the C2 and C5 positions are about equally favorable. Although the C5 species is generally identified as the experimentally observed product in aqueous media for both protonated and unprotonated imidazole, some experimental evidence exists indicating the presence of other forms. Our results suggest that this other form is the C2 adduct. The calculations also point to the protonated form of imidazole being less reactive than imidazole, which is in accord with experimental observations.

AbInitio SCF MO calculations on the reaction of hydroxyl radical with cytosine

“…Although more information exists regarding reactions of OH with thymine and uracil, the greater experimental controversy for site preference of OH addition to cytosine lent impetus to the choice of cytosine as a subject for these calculations. A recent paper (8) suggested that site selection of OH attack of cytosine is more discriminating than for the analogous reactions with thymine and uracil and that the major product, in opposition to expectations from the results of our previous studies on OH reactions with N heterocycles (1,2), was the species favored on account of electrophilic interactions rather than being the most electronically delocalized product. These factors contributed to the selection of cytosine as the subject for study.…”

“…The more extensively electron delocalized C6 adduct is calculated to be more stable relative to the C4 adduct and the experimentally found C5 adduct. In earlier theoretical studies (2,22,(26)(27)(28), an arbitrary value of 40 kJ/mol was set as the limit of significant energy difference; thus, the small energy difference between the C4 and C5 adducts (5 kJ/mol) suggests similar stabilities, and no conclusive ordering in their relative stabilities can be made.…”

“…adducts suggests that an extension of the basis set would not overturn our conclusions. The geometry optimization for the most important species has been canied out to a uniform level of accuracy, higher than that routinely enforced in ab initio studies (vide infra) Initial geometries for both cytosine and species 2-7 were obtained from X-ray diffraction data (21) with the local geometry of the OH addition site assumed to be tetrahedral (1,2,5,6). Initial geometries of species 9 and 10 were taken from modified optimized geometries of the analogous keto forms of the adducts.…”

“…Experimental results on the reaction of OH with cytosine agree that ring addition occurs (3,4), but information on site preference for addition is obtained only by indirect methods (esr, pulse radiolysis) leaving results open to some controversy. Ab initio calculations, however, provide a direct aid to identification of favored product isomers, and have been shown to be useful in past studies, supplementing experimental results (1,2,5,6).…”

“…The products resulting from such reactions are depicted in Fig. 1; total energies and energy changes for the reactions [l] and [2] have been determined. Calculations were also performed on different tautometers as well as on various internally H-bonded forms of the adducts, and kinetic aspects of the reaction were investiated via the calculation of assumed reaction paths.…”