Radicals formed by electron transfer from cytosine and 1-methylcytosine to the triplet state of anthraquinone-2,6-disulfonic acid. A Fourier-transform EPR study

Abstract

Radicals generated by electron transfer from cytosine and 1-methylcytosine to the laser-induced triplet state of anthraquinone-2,6-disulfonic acid were studied by time-resolved Fourier-transform (FT) EPR in H₂O and D₂O at 10°C. The main products observed on the nanosecond timescale were successors of the base radical cations. Their hyperfine couplings were determined by computer simulation of the experimental spectra. Assignment of the radical structures was supported by density functional theory (DFT) quantum mechanical calculations. The experiments with cytosine deprotonation at N1 resulted in the cytosin-1-yl radical 1 with high spin density at N1 and C5 whereas, for 1-methylcytosine, deprotonation at the exocyclic amino group yielded the aminyl radical 2. For both parent compounds, cytosine and 1-methylcytosine, two additional long-lived radicals (3 and 4) with unknown structure were detected on the nanosecond to microsecond timescale. Their spectral parameters were independent of the N1 substituent.