Lifetimes and Reaction
Pathways of Guanine Radical
Cations and Neutral Guanine Radicals in an Oligonucleotide in Aqueous
Solutions
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Abstract
The exposure of guanine in the oligonucleotide 5′-d(TCGCT)
to one-electron oxidants leads initially to the formation of the guanine
radical cation G<sup>•+</sup>, its deptotonation product G(-H)<sup>•</sup>, and, ultimately, various two- and four-electron oxidation
products via pathways that depend on the oxidants and reaction conditions.
We utilized single or successive multiple laser pulses (308 nm, 1
Hz rate) to generate the oxidants CO<sub>3</sub><sup>•–</sup> and SO<sub>4</sub><sup>•–</sup> (via the photolysis
of S<sub>2</sub>O<sub>8</sub><sup>2–</sup> in aqueous solutions
in the presence and absence of bicarbonate, respectively) at concentrations/pulse
that were ∼20-fold lower than the concentration of 5′-d(TCGCT).
Time-resolved absorption spectroscopy measurements following single-pulse
excitation show that the G<sup>•+</sup> radical (p<i>K</i><sub>a</sub> = 3.9) can be observed only at low pH and is hydrated
within 3 ms at pH 2.5, thus forming the two-electron oxidation
product 8-oxo-7,8-dihydroguanosine (8-oxoG). At neutral pH, and single
pulse excitation, the principal reactive intermediate is G(-H)<sup>•</sup>, which, at best, reacts only slowly with H<sub>2</sub>O and lives for ∼70 ms in the absence of oxidants/other radicals
to form base sequence-dependent intrastrand cross-links via the nucleophilic
addition of N3-thymidine to C8-guanine (5′-G*CT* and 5′-T*CG*).
Alternatively, G(-H)<sup>•</sup> can be oxidized further by
reaction with CO<sub>3</sub><sup>•–</sup>, generating
the two-electron oxidation products 8-oxoG (C8 addition) and 5-carboxamido-5-formamido-2-iminohydantoin
(2Ih, by C5 addition). The four-electron oxidation products, guanidinohydantoin
(Gh) and spiroiminodihydantoin (Sp), appear only after a second (or
more) laser pulse. The levels of all products, except 8-oxoG, which
remains at a low constant value, increase with the number of laser
pulses