Electron Transfer between Guanosine Radical and Amino Acids in Aqueous Solution. 1. Reduction of Guanosine Radical by Tyrosine

Abstract

As a model of chemical DNA repair, the reductive electron transfer from the aromatic amino acid tyrosine to the radical of the purine base guanosine monophosphate (GMP) was studied by time-resolved chemically induced dynamic nuclear polarization (CIDNP). The guanosyl radicals were photochemically generated in the quenching reaction of the triplet excited dye 2,2'-dipyridyl. Depending on the pH of the aqueous solution, four different guanosyl radicals were observed. The identification of the radicals was possible because of the high sensitivity of CIDNP to distinguish them through their ability or disability of participating in the degenerate electron hopping reaction with the diamagnetic molecules of guanosine monophosphate in the ground state. The CIDNP kinetics in this three-component system containing the dye, GMP, and N-acetyl tyrosine is strongly dependent on the efficiency of the electron-transfer reaction from tyrosine to the nucleotide radical. Quantitative analysis of the CIDNP kinetics obtained at different concentrations of the amino acid, together with the comparison with the CIDNP kinetics of the two-component systems (dipyridyl/tyrosine and dipyridyl/GMP) allowed for the determination of the rate constant ke of the reductive electron-transfer reaction for five pairs of reactants, with different protonation states depending on the pH: GH++*/TyrOH (pH 1.3), G+*/TyrOH (pH 2.9), G(-H)*/TyrOH (pH 7.5), G(-H)*/TyrO- (pH 11.3), and G(-2H)-*/TyrO- (pH 13.3). The rate constant ke varies from (7.1 +/- 3.0) x 10(8) M-1 s-1 (pH 1.3, 2.9) to less than 6 x 10(6) M-1 s-1 (pH 13.3).

Photoinduced Proton‐Coupled Electron Transfers in Biorelevant Phenolic Systems

“…In addition to PSII (24)(25)(26) and mimicking strategies developed for energy issues, the tyrosyl radical appears to be the most prominent amino acid radical involved in enzymatic PCET mechanisms, such as those encountered in ribonucleotide reductase (RNR) (27)(28)(29)(30), prostaglandin H synthase (31)(32)(33)(34), galactose oxidase (35)(36)(37), adenosylcobalamin-dependent enzymes (38,39), cytochrome c oxidase (CcO) (40)(41)(42) or amine synthase (43). It is indeed now known or in some cases suspected that these radicals play a crucial role in DNA damage and repair (see for example [44][45][46]). The interest for phenolic compounds is also related to their antioxidant properties (47)(48)(49)(50).…”

Synthesis of nucleotide–amino acid conjugates designed for photo-CIDNP experiments by a phosphotriester approach

“…Based on the above mentioned reasons, we have chosen 4-benzoylbenzoic acid, 2’-deoxyguanosine, and L-tryptophan [7,9,13–20] as the dye, nucleoside, and amino acid building blocks, respectively, to construct conjugates 1 – 8 (Fig. 1) linked in the uniform manner through the phosphate groups.…”

“…Although there is a number of well-developed methods of the automatic solid phase supported synthesis (SPSS) of oligonucleotide–peptide conjugates [8], this strategy does not ensure the availability of the model conjugates for TR NMR photo-CIDNP experiments due to the lack of versatility and difficulties in scaling up the process. As shown in [9], at least 0.01–0.03 mmol of a compound is required for each photo-CIDNP experiment.…”

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

“…[33] Briefly, samples in standard 5 mm Pyrex NMR tubes placed in the probe of a Bruker Avance III 400 MHz NMR spectrometer were irradiated by a Coherent Compex Pro 102 XeCl excimer laser (wavelength 308 nm, output pulse energy used 80 mJ). [33] Briefly, samples in standard 5 mm Pyrex NMR tubes placed in the probe of a Bruker Avance III 400 MHz NMR spectrometer were irradiated by a Coherent Compex Pro 102 XeCl excimer laser (wavelength 308 nm, output pulse energy used 80 mJ).…”