A Phototautomerizable Model DNA Base Pair

2-(2‘-Hydroxyphenyl)benzoxazole (HBO) undergoes rapid photoinduced proton transfer from the enol-imine to the keto-amine tautomer. When incorporated in duplex DNA opposite an abasic site, HBO appears to be a good mimic of a natural DNA base pair based on duplex stability, UV and CD spectroscopy, and molecular dynamics simulations. In the chosen sequence context, HBO exists exclusively as the syn -enol tautomer, which is consistent with an environment of the model base that is dominated by neighboring nucleobases and not by solvation. The syn -enol efficiently undergoes photoinduced proton transfer to the keto-tautomer, which should allow for the study of tautomerization within the duplex DNA.