pH-Dependent Equilibrium between 5-Guanidinohydantoin and Iminoallantoin Affects Nucleotide Insertion Opposite the DNA Lesion.

Four-electron oxidation of 2'-deoxyguanosine (dG) yields 5-guanidinohydantoin (dGh) as a product. Previously, we hypothesized that dGh could isomerize to iminoallantoin (dIa) via a mechanism similar to the isomerization of allantoin. The isomerization reaction was monitored by HPLC and found to be pH dependent with a transition pH = 10.1 in which dGh was favored at low pH and dIa was favored at high pH. The structures for these isomers were confirmed by UV-vis, MS, and (1)H and (13)C NMR. Additionally, the UV-vis and NMR experimental results are supported by density functional theory calculations. A mechanism is proposed to support the pH dependency of the isomerization reaction. Next, we noted the hydantoin ring of dGh mimics thymine, while the iminohydantoin ring of dIa mimics cytosine; consequently, a dGh/dIa site was synthesized in a DNA template strand, and standing start primer extension studies were conducted with Klenow fragment exo(-). The dATP/dGTP insertion ratio opposite the dGh/dIa site as a function of pH was evaluated from pH 6.5-9.0. At pH 6.5, only dATP was inserted, but as the pH increased to 9.0, the amount of dGTP insertion steadily increased. This observation supports dGh to dIa isomerization in DNA with a transition pH of ∼8.2.