Steric crowding governs the structural distortions in the lesion-containing duplexes. For the 14R (+)-adduct, steric crowding between the hydroxyl group O12 and 5’-flanking base C5 (atom C6) produces the enlarged Rise. For the 14S (–)-adduct, the crowding between the hydroxyl O12, pointing toward the 3’-side of the lesion, and the 3’-flanking base C7 (atom N4) causes the untwisting. The hydroxyl O12, C6 atom of the C5 base in the 14R (+)-adduct, and the N4 atom of the C7 base in the 14S (–)-adduct are shown in semi-transparent CPK. See also Movies S3 and S4, Supporting Information.