Chemistry and DNA Alkylation Reactions of Aziridinyl Quinones: Development of an Efficient Alkylating Agent of the Phosphate Backbone

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Abstract
Described herein are detailed hydrolytic studies of a series of aziridinyl quinones, which trap nucleophiles when protonated. This study provided a compilation of the rate constants for nucleophile trapping and of the pKa values for the protonated aziridinyl quinones. A linear free energy relationship, including the antitumor agent DZQ, as well as other synthetic quinone derivatives, was obtained as a result of this study. Protonated DZQ has the relatively high pKa value of 3.8, which explains the enhanced cross-linking of DNA by DZQ and other related aziridinyl quinones at pH 4. The literature often shows aziridinyl quinone protonation occurring at the aziridinyl nitrogen, but the dependence of pKa values on quinone substituents indicates the presence of delocalization, which must arise from O-protonation. Also investigated were the DNA alkylation reactions of protonated aziridinyl quinones. At the outset of this study, we postulated that these “hard” electrophiles would alkylate the phosphate backbone of DNA. Bulk DNA is up to 35% alkylated by protonated aziridinyl quinones as judged by the incorporation of the quinone chromophore into the DNA. The presence of phosphate alkylation was verified by a 1H−31P NMR correlation experiment with DZQ-alkylated hexamer. Our modeling studies present a new picture of DZQ alkylation of DNA, where there is competition between N(7) and phosphate alkylation. The conclusions of this part of our study are that the phosphate backbone should be considered as a possible target of any DNA-alkylating agent and that an assessment of phosphate alkylation is best made with a 1H−31P NMR correlation experiment. Finally, the benzimidazole-based aziridinyl quinone 2 was observed to undergo aziridine ring opening followed by hydrolytic removal of the aminoethyl group from the quinone ring. This reaction was used to tag the phosphate backbone of DNA with aminoethyl groups. Such tags render anionic phosphates cationic and could also be employed as points of attachment for chromophores, spin labels, or other moieties to DNA.