Changes of electrostatic potential around the DNA molecule resulting from chemical modifications of nucleotides may play a role in enzymatic recognition of damaged sites. Effects of chemical modifications of nucleotides on the structure of DNA have been characterized through electronic structure computations. Quantum mechanical structural optimizations of fragments of five pairs of nucleotides with thymine or thymine glycol were performed at the density functional level of theory with a B 3LYP exchange-correlation functional and 6-31G(d,p) basis sets. The electrostatic potential (EP) around DNA fragments was projected on a cylindrical surface around the double helix. The 2D maps of EP of intact and damaged DNA fragments were compared using image analysis methods to identify and measure modifications of the EP that result from the occurrence of thymine glycol. It was found that distortions of phosphate groups and displacements of the accompanying countercations by up to ~0.5 Âalong the axis of DNA are clearly reflected in the EP maps. Modifications of the EP in the major groove of DNA near the damaged site are also reported. © 2008 American Chemical Society.