Density functional theory is used to;calculate the electronic structure of the neutral flavin radical, FADE, formed in the light-induced electron-transfer reaction of DNA repair in cis,syn-cyclobutane pyrimidine dimer photolyases. Using the hybrid B3LYP:functional together with the double-zeta basis set; EPR-II H-1, C-13, N-15, and O-17 isotropic and anisotropic hyperfine couplings are calculated and explained by reference to the electron densities of the highest occupied molecular orbital and of the unpaired spin distribution on the radical, Comparison of calculated and/experimental hyperfine couplings obtained from EPR and ENDOR/TRIPLE resonance leads to a refined structure for the FAD cofactor in Escherichia coli DNA photolyase: Hydrogen bonding at N3H, O4, and N5H results in significant changes in the unpaired spin density distribution and hyperfine coupling constants. The calculated electronic structure of FADH(.) provides evidence for a superexchange-mediated electron transfer between the cyclobutane pyrimidine dimer lesion and the 7,8-dimethyl isoalloxazine moiety of the flavin cofactor via the adenine; moiety.