ESR and ENDOR study of 5′dCMP at 6 K

The ESR spectra of single crystals of deoxycytidine 5′-phosphate⋅H2O (5′dCMP), x irradiated at 11 K and observed at 6 K, exhibit signals from three distinct free radicals. Analysis of the ENDOR spectra from two of these radicals indicates that these result from oxidation and reduction of the cytosine base. The reduced species exhibits hyperfine coupling to the C6–Hα proton (Aiso(H6) =−1.392 mT). The oxidized species exhibits hyperfine couplings to the C5–Hα proton (Aiso(H5) =−1.495 mT), N1 and N3, (Aiso(N1) ∼0.5 mT), (Aiso(N3) ∼0.25 mT) and to the C1′–Hβ proton (Aiso(Hβ) =1.483 mT). The third free radical has ESR spectra characteristic of an oxygen centered defect and is thought to be a hole located primarily on the C3′–O3′ oxygen, but shared with a neighboring HO–PO−3−CH2- group.