Diazonium ions are reactive intermediates derived by metabolites of carcinogenic nitrosamines. They can react with DNA at many heteroatoms and form different adducts, which might eventually lead to cancer. Compared to other classical organic alkylating agents, simple diazonium ions react more selectively at oxygen atoms of nucleobases. The resulting oxygen adducts have been shown to be correlated with the mutagenicity of the parent nitrosamines. Therefore, it is of great importance to understand the factors that account for this preference of diazonium ions for oxygen atoms of nucleobases.

This work was carried out to study the site selectivity in DNA alkylation by primary diazonium ions using 1-propyldiazonium ion as a tool. The 1-propyldiazonium ion can generate both n-propylation and iso-propylation products. The yield of iso-propylation and the ratio of n-propylation to iso-propylation at a given heteroatom are roughly measurements of the association constant and nucleophilicity of the site to the diazonium ion, respectively. Therefore, the individual effect of the association constants and nulceophilicities on the product yields can be discussed.

Alkylation of DNA as well as its nucleoside monomers by 1-propyldiazonium ion was carried out. After hydrolysis, the percent yields of n-propylation and iso-propylation products were determined by HPLC and LC/MS. The variation in the iso-propylation yields in nucleoside reactions is small and relatively random. In DNA, however, preferential iso-propylation in the minor groove was observed, indicating that the double helix favors the association of 1-propyldiazonium in the minor groove. In the case of the ratio of n-propylation to iso-propylation, the variation is also small in nucleoside reactions. In DNA, however, the ratios of n-propylation to iso-propylation for some sites show significant enhancement, while the others show much smaller enhancement or remain almost the same as compared to the nucleosides. These results demonstrate that the double helix selectively enhances the nucleophilicities of certain sites. Thus, it is the structure of DNA that dictates the atom site selectivity in DNA alkylation by primary diazonium ions.