Radicals derived from uric acid and its methyl derivatives in aqueous solution: an EPR spectroscopy and theoretical study

The oxidation of uric acid and of four N-methyluric acids in aqueous solution was studied by EPR spectroscopy. The primary oxidising radicals react with uric acid and its methyl derivatives by formal hydrogen abstraction from an NH group to yield radical-anions in neutral or moderately basic solutions and the respective radical-dianions in basic media. In the case of uric acid, the radical-trianion was detected at very high pH. The pKa values of the radical-anions were determined to be in the range 9.5–11.2. The pKa of uric acid radical-dianion was estimated to be 13.0. DFT calculations were performed to assign the hyperfine coupling constants and to determine the predominant tautomeric structure of the radicals. The uric acid radical-anion exists as the N1H, N9H tautomer, while in the radical-dianion the N1H structure is the most stable one. The intrinsic acidity of the NH protons both in uric acid and in its radicals seems to follow the order N1H < N9H < N3H