Electron transfer reactions between nucleic acid residues in DNA and strong oxidants are often the critical initial steps that initiate oxidative, irreversible DNA damage. Employing laser Ñash photolysis transient absorption spectroscopic techniques, we investigated the characteristics of electron transfer reactions in aqueous solutions between the 2@-deoxynucleoside 5@-monophosphates, dGMP, dAMP, dCMP and dTMP and a representative one-electron oxidant. The latter was a radical cation of a pyrene derivative with enhanced water-solubility, 7,8,9,10-tetrahydroxytetrahydrobenzo[a]pyrene (BPT). The BPT radical cation was BPT~`, generated by intense nanosecond laser pulse (308 or 355 nm, 50È70 mJ pulse~1 cm~2) by a non-linear consecutive two-photon absorption process. No electron transfer reactions were observed with dAMP, dTMP and dCMP, consistent with their unfavorable redox potentials. However, efficiently oxidized dGMP BPT~ẁ ith a rate constant M~1 s~1, which is smaller than the di †usion-controlled value by a k b \ 1.7 ^0.1) ] 109 factor of only D3. The dGMP(-H) neutral radicals formed on time scales of a few microseconds, were ĩdentiÐed by their characteristic transient absorption spectrum mn). The rate constant of electron (j max D 310 transfer from dGMP to was smaller in by a factor of D1.5 than in This kinetic isotope BPT~`D 2 O H 2 O. e †ect indicates that the electron transfer reaction from dGMP to is accompanied by the deprotonation BPT~ò fand therefore appears to be a proton-coupled electron transfer reaction. dGMP~`,

Equilenin, an important component of a widely prescribed hormone replacement formulation for postmenopausal women, is metabolized by mammalian P450 enzymes to the catechol 4-hydroxyequilenin (4-OHEN). The oxidized o-quinone derivative of 4-OHEN is known to form cyclic covalent adducts with DNA [Bolton, J. (1998) Chem. Res. Toxicol. 11, 1113] in vitro and in vivo. The characteristics of 4-OHEN-DNA adduct formation were investigated with the oligonucleotides 5'-d(CCATCGCTACC) (I), its complementary strand 5'-d(GGTAGCGATGG) (II), one rich in C and the other in G, and the duplexes I.II. The identities of the modified bases were elucidated in terms of four stereoisomeric 4-OHEN-2'-deoxynucleoside standards described earlier [Shen et al. (2001) Chem. Res. Toxicol. 11, 94; Embrechts et al. J. Mass Spectrom. 36, 317). The reactions of 4-OHEN with C are favored overwhelmingly in both single-stranded I and II with no guanine adducts observed in either case, and only minor proportions of A adducts were detected in sequence II. However, guanine adducts are observed in oligonucleotides that contain only G and unreactive T residues. The relative levels of cyclic covalent adducts observed in single-stranded I, II, and duplex I.II are approximately 54:21:5, with only the end C groups in I modified in the I.II duplex. When 4-OHEN is reacted with calf thymus DNA, the reaction yield of cyclic adducts is more than approximately 10(3)-fold lower than in I. The cyclic 4-OHEN adducts lead to a pronounced thermal destabilization of duplexes I.II. Overall, cyclic adduct formation is markedly dependent on the sequence context and secondary structure of the DNA. The latter effect is attributed to the poor accessibilities of 4-OHEN to the reactive nucleotide Watson-Crick hydrogen-bonding interface in the interior of the duplex. In the single-stranded oligonucleotides I and II, the strikingly different selectivities of adduct formation are attributed to the formation of noncovalent preassociation complexes that favor reaction geometries with C, rather than with A or G. Finally, the levels of several typical biomarkers of oxidative DNA damage (including 8-oxo-2'-deoxyguanosine) are formed in I in aqueous solutions with a yield at least 10 times smaller than the yield of cyclic 4-OHEN-dC adducts under identical reaction conditions.

The carcinogenic and mutagenic benzo[a]pyrenediol epoxide derivative 7r,8t-dihydroxy-9r,10/-epoxy-7,8,9,-10-tetrahydrobenzo[a] pyrene (BPDE) binds via its C-10 position predominantly to the exocyclic amino group of guanine residues in native DNA. In such DNA adducts, the fluorescence of the pyrenyl moieties is strongly quenched by physicochemical interaction with the DNA bases. Using nanosecond time scale transient absorption techniques,