EPR-detection of the guanosyl radical cation in aqueous solution. Quantum chemically supported assignment of nitrogen and proton hyperfine couplings

In situ photolysis of aqueous solutions containing either 2&#x0301;-deoxyguanosine-5&#x0301;-phosphate or guanosine-2&#x0301;-phosphate and K₂S₂O₈ at pH<4 resulted in a resolved EPR-spectrum consisting of 40 equidistant peaks with a separation of 0.068 mT centered at g=2.0038. It originates from the guanosyl radical cation generated upon one-electron oxidation of the base moiety by SO^-•₄. In order to assist the assignment of the EPR-spectrum theoretical nitrogen and hydrogen hyperfine coupling constants were employed. They were obtained from spin density calculations based on the restricted open shell formalism at an ab initio level. For a conversion of calculated spin densities into coupling constants, calibration curves were constructed by applying a McConnell type approach to known nitrogen heterocyclic radical cations. For four different tautomers of the guanosyl radical cation theoretical spin densities and calibration curves were used to obtain coupling constants. They were applied in a simulation of the experimental EPR-spectrum. The simulation in combination with the calculated energies of the tautomers indicate that the EPR-spectrum originates from the guanosyl radical cation protonated at NI.