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Kinetics of Reversible Protonation of Transient Neutral Guanine Radical in Neutral Aqueous Solution
Author(s) -
Morozova Olga B.,
Fishman Natalya N.,
Yurkovskaya Alexandra V.
Publication year - 2018
Publication title -
chemphyschem
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.016
H-Index - 140
eISSN - 1439-7641
pISSN - 1439-4235
DOI - 10.1002/cphc.201800539
Subject(s) - cidnp , chemistry , protonation , photochemistry , radical , radical ion , deprotonation , guanine , kinetics , ketyl , hydroxyl radical , benzophenone , ion , organic chemistry , nucleotide , biochemistry , physics , quantum mechanics , gene
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 p K a =8.0±0.2 of the cation radical (G .+ )′.