Oxidation of Guanine in G, GG, and GGG Sequence Contexts by Aromatic Pyrenyl Radical Cations and Carbonate Radical Anions:  Relationship between Kinetics and Distribution of Alkali-Labile Lesions

Abstract

Oxidatively generated DNA damage induced by the aromatic radical cation of the pyrene derivative 7,8,9,10-tetrahydroxytetrahydrobenzo[a]pyrene (BPT), and by carbonate radicals anions, was monitored from the initial one-electron transfer, or hole injection step, to the formation of hot alkali-labile chemical end-products monitored by gel electrophoresis. The fractions of BPT molecules bound to double-stranded 20-35-mer oligonucleotdes with noncontiguous guanines G, and grouped as contiguous GG and GGG sequences, were determined by a fluorescence quenching method. Utilizing intense nanosecond 355 nm Nd: Yag laser pulses, the DNA-bound BPT molecules were photoionized to BPT •+ radicals by a consecutive two-photon ionization mechanism. The BPT •+ radicals thus generated within the duplexes selectively oxidize guanine by intraduplex electron transfer reactions, and the rate constants of these reactions follow the trend 5′-..GGG.. > 5′-..GG.. > 5′-..G… In the case of CO 3 •− radicals, the oxidation of guanine occurs by intermolecular collision pathways and the bimolecular rate constants are independent of base sequence context. However, the distributions of the end-products generated by CO 3 •− radicals, as well as by BPT •+ , is base sequence context-dependent and is greater than in isolated guanines at the 5′-G in 5′-…GG… sequences, and the first two 5′-guanines in the 5′-..GGG sequences. These results help to clarify the conditions that lead to a similar or different base sequence dependence of the initial hole injection step and the final distribution of oxidized, alkalilabile guanine products. In the case of the intermolecular one-electron oxidant CO 3 •− , the rate constant of hole injection is similar for contiguous and isolated guanines, but the subsequent equilibration of holes by hopping favors trapping and product formation at contiguous guanines, and the sequence dependence of these two phenomena are not correlated. In contrast, in the case of the DNA bound oxidant BPT •+ , the hole injection rate constants, as well as hole equilibration, exhibit a similar dependence on base sequence context, and are thus correlated to one another.

Applying Epigenetics to Alzheimer’s Disease via the Latent Early–life Associated Regulation (LEARn) Model

“…DNA oxidation usually occurs at guanine dimers or trimers and is associated with lesions of the DNA strand [80]. It interferes with binding of transcription factors, including the specificity protein (SP) family [55,81], although it does not appear to reduce nuclear factor B (NF-B) binding [55].…”

Prototropic equilibria in DNA containing one-electron oxidized GC: intra-duplex vs. duplex to solvent deprotonation

“…, GG, GGG, are ultimately the most stable hole localization sites after long range hole transfer and these G stacks form loci for DNA base damage. 2 – 6, 12 – 16 …”

Oxidative Processes and Antioxidative Metaloenzymes

“…The most basic reaction is one-electron oxidation, the result of which is essentially independent of the process by which it is oxidized [110]. The loss of an electron converts DNA to its radical cation (an electron "hole"), which migrates reversibly through duplex DNA by hopping until it is trapped in an irreversible chemical reaction to form a structurally modified base [95].…”