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.

. 64,914 (1986).Ab initio calculations have been carried out on the relative stabilities of various possible products of the reaction between cytosine and the OH radical. These products are of importance in modelling radiation damage to living tissues. The preferred theoretical gas-phase addition site is the C6 ring atom according to these calculations. The analysis of a series of possible contributions to solvent effects strongly suggests the predominance of intermolecular H bonds in stabilizing the experimentally observed C5 adduct.BEVERLEY G. EAToCK, WILLIAM L. WALTZ, et PAUL G. MEZEY. Can. J. Chem. 64,914 (1986).On a effectut de calculs ab initio sur les stabilitks relatives de divers produits possibles pour la rkaction de la cytosine avec le radical OH. Ces produits sont importants dans l'klaboration d'un modkle pour les dommages dfis B la radiation dans les tissus vivants. D'apr2s ces calculs thtoriques, le site prkftrt pour l'addition en phase gazeuse est l'atome de carbone C6. L'analyse d'une skrie de contributions possibles aux effets de solvant sugg2re fortement la prkdominance de liaisons hydrogknes intermoltculaires dans la stabilisation de l'adduit en C5 qui est observk expkrimentalement.[Traduit par la revue] Introduction Radiation damage to nitrogen heterocycles has attracted interest due to the relevance of such effects for biologically important systems. As an extension of studies on the reactions of OH radicals with N heterocyclic systems (pyridine, imidazole) (1, 2), we have investigated the attack of OH on the pyrimidine base cytosine. 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).Radiation damage to cytosine is of interest because of its status as a component of DNA. Consideration of the N bases of DNA as subjects for ab initio studies favors the pyrimidines, both because of their greater susceptibility to OH attack (7) and the simpler problem that the electronic structure of the pyrimidines represents, as compared to the purine bases. 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 factor...