In this work, iminyl sigma-radical formation in several one-electron-oxidized cytosine analogs, including 1-MeC, cidofovir, 2'-deoxycytidine (dCyd), and 2'-deoxycytidine 5'-monophosphate (5'-dCMP), were investigated in homogeneous, aqueous (D2O or H2O) glassy solutions at low temperatures by employing electron spin resonance (ESR) spectroscopy. Upon employing density functional theory (DFT) (DFT/B3LYP/6-31G* method), the calculated hyperfine coupling constant (HFCC) values of iminyl a-radical agree quite well with the experimentally observed ones, thus confirming its assignment. ESR and DFT studies show that the cytosine iminyl a-radical is a tautomer of the deprotonated cytosine pi-cation radical [cytosine pi-aminyl radical, C(N4-H)(center dot)]. Employing 1-MeC samples at various pHs ranging from ca. 8 to 11, ESR studies show that the tautomeric equilibrium between C(N4-H)(center dot) and the iminyl sigma-radical at low temperature is too slow to be established without added base. ESR and DFT studies agree that, in the iminyl sigma-radical, the unpaired spin is localized on the exocyclic nitrogen (N4) in an in-plane pure p-orbital. This gives rise to an anisotropic nitrogen hyperfine coupling (A(zz) = 40 G) from N4 and a near isotropic beta-nitrogen coupling of 9.7 G from the cytosine ring nitrogen at N3. Iminyl sigma-radical should exist in its N3-protonated form, as the N3-protonated iminyl sigma-radical is stabilized in solution by over 30 kcal/mol (Delta G = -32 kcal/mol) over its conjugate base, the N3-deprotonated form. This is the first observation of an isotropic beta-hyperfine ring nitrogen coupling in an N-centered DNA radical. Our theoretical calculations predict that the cytosine iminyl sigma-radical can be formed in double-stranded DNA by a radiation-induced ionization-deprotonation process that is only 10 kcal/mol above the lowest energy path.