Representative 2Fo − Fc electron-density maps. (a) Base pairs of T14 and C15 of chain C with G6 and A7 of chain D from the 19OR DNA. (b) Base pairs between chain G (C7 and T8) and chain H (A18 and G19) from the 25OL DNA. Hydrogen bonds are shown as dashed lines. 2Fo − Fc electron-density maps are contoured at 0.16 and 0.32 e Å−3 for 19OR and 25OL, respectively. The images were generated using PyMOL.