Kinetics of Reversible Protonation of Transient Neutral Guanine Radical in Neutral Aqueous Solution

Time‐resolved chemically induced dynamic nuclear polarization (TR‐CIDNP) is applied to follow transformation of the short‐lived neutral guanine radical into a secondary guanine radical by its protonation, presumably at position N7. In the initial step the photoreaction of guanosine‐5′‐monophosphate...

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Bibliographic Details
Published in:Chemphyschem 2018-10, Vol.19 (20), p.2696-2702
Main Authors: Morozova, Olga B., Fishman, Natalya N., Yurkovskaya, Alexandra V.
Format: Article
Language:English
Subjects:
Cations
DNA oxidation
guanine radical
Magnetic resonance
NMR spectroscopy
Organic chemistry
photo-oxidation
photochemistry
Protonation
Transformations
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Summary:Time‐resolved chemically induced dynamic nuclear polarization (TR‐CIDNP) is applied to follow transformation of the short‐lived neutral guanine radical into a secondary guanine radical by its protonation, presumably at position N7. In the initial step the photoreaction of guanosine‐5′‐monophosphate (GMP) with triplet excited 3,3′,4,4′‐tetracarboxy benzophenone (TCBP) leads to formation of the neutral radical G(−H).. The evidence of the radical conversion is based on the inversion of CIDNP sign for TCBP and GMP protons on the microsecond timescale as a result of the change in magnetic resonance parameters in the pairs of TCBP and GMP radicals due to structural changes of the GMP radical. Acceleration of the CIDNP sign change upon addition of phosphate (proton donor) confirms that the radical transformation responsible for the observed CIDNP kinetics is protonation of the neutral guanine radical with formation of the newly characterized cation radical, (G.+)′. From the full analysis of the pH‐dependent CIDNP kinetics, the protonation and deprotonation behaviour is quantitatively characterized, giving pKa=8.0±0.2 of the cation radical (G.+)′. An NMR study of the CIDNP kinetic data obtained for a reversible photocycle involving short‐lived guanine radicals in neutral and slightly basic aqueous solution results in the quantitative characterization of the pH‐dependent rates of transformation of the neutral guanine radical via protonation at the N7 position, enabling the calculation of the pKa value of the newly established transient guanine cation radical.
ISSN:1439-4235
1439-7641
DOI:10.1002/cphc.201800539