posted on 2016-02-20, 09:54authored byLaura Banu, Voislav Blagojevic, Diethard K. Bohme
The oxidation of guanine was investigated in water/methanol
solution
both in the absence and in the presence of Pb(II) with a variable
temperature reactor coupled to a tandem mass spectrometer that allowed
signature ions of solution reagents and products to be monitored by
electrospray ionization (ESI). Two different oxidizing agents were
employed, one strong (peroxymonosulfuric acid) and one weaker (hydrogen
peroxide). Peroxymonosulfuric acid was observed to oxidize guanine
rapidly at room temperature, kapp >
10–2 s–1, whether in the absence
or
in the presence of Pb(II), to produce spiroiminohydantoin. Guanine
did not show measurable oxidation by hydrogen peroxide in the absence
of Pb(II) at concentrations of H2O2 up to 1
M at temperatures up to 333 K (kapp <
3 × 10–8 s–1 at 298 K), but
in the presence of Pb(II), it was observed to produce both 5-carboxamido-5-formamido-2-iminohydantoin
(2-Ih) and imidazolone (Iz) in a ratio of 2.3 ± 0.1 with a total
rate enhancement of more than 4 × 103. The activation
energy was measured to be 82 ± 11 kJ mol–1 and
is more than 120 kJ mol–1 lower than that for the
uncatalyzed oxidation with hydrogen peroxide measured to be at least
208 ± 26 kJ mol–1. An activation energy of
113 ± 9 kJ mol–1 has been reported by Bruskov
et al. (Nucleic Acids Res.2002, 30, 1354) for the heat-induced oxidation by hydrogen peroxide
of guanine embedded as guanosine in DNA which leads to the production
of 8-oxo-7,8-dihydro-guanine (8-oxo-Gua). The atomic lead dication
lowers the activation energy by activating the hydrogen peroxide oxidant,
possibly by O–O bond activation, and by directing the oxidation,
possibly through coordination to the functional groups adjacent to
the carbon C5: the C6 carbonyl group and the N7 nitrogen. The coupling
of tandem mass spectrometry (MS2) with a simple variable
temperature reactor by ESI proved to be very effective for measuring
reaction kinetics and activation energies in solution. Signature ions
of both reagents and products, as well as the catalyst, could be identified,
and the data were acquired in real time. The technique should be suitable
for exploring other chemical and biochemical reactions that occur
on similar time scales (minutes to hours).