Selective abstraction of 2H from C-5' of thymidylate in an oligodeoxynucleotide by the radical center at C-6 of the diradical species of neocarzinostatin: chemical evidence for the structure of the activated drug-DNA complex.

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Abstract
Use has been made of the mechanism of DNA deoxyribose damage by the ene-diyne-containing chromophore of the antitumor antibiotic neocarzinostatin to provide chemical evidence for the structure of the activated drug-DNA complex. Radical centers at C-2 and C-6 of the diradical form of the glutathione-activated chromophore abstract hydrogen atoms from C-1' of the C residue and C-5' of the T residue in AGC.GCT to generate a bistranded lesion consisting of an abasic site at C and a strand break at T. This laboratory has proposed a molecular model for the drug-DNA interaction in which the naphthoate moiety of the chromophore intercalates between A.T and G.C, placing the diradical core in the minor groove, so that the radical centers at C-6 and C-2 are close to C-5' of T and C-1' of C, respectively. To determine which radical center abstracts one of the hydrogen atoms from C-5', the self-complementary oligodeoxynucleotide GCAGCGCTGC was synthesized with 2H at both 5' positions of the T residue and treated with glutathione-activated chromophore. Sequencing-gel electrophoresis showed that drug attack was limited to the T and C residues and that abstraction of 2H from C-5' exhibited a small isotope selection effect of 1.25. 1H NMR spectroscopic examination of the reacted chromophore, isolated by HPLC, indicated that 2H was selectively abstracted by C-6, providing experimental corroboration of the model and further elucidating the chemical mechanism. Since direct strand breakage at the T residue exceeds (44% more) abasic site formation at the C residue, other models of drug-DNA interaction leading to only single-strand breaks are also considered.