Photoinduced electron transfer in 5-bromouracil labeled DNA. A contrathermodynamic mechanism revisited by electron transfer theories.

The understanding of the 5-bromouracil (BrU) based photosensitization mechanism of DNA damage is of large interest due to the potential applications in photodynamic therapy. Photoinduced electron transfer (ET) in BrU labeled duplexes comprising the 5'-GBrU or 5'-ABrU sequence showed that a much lower reactivity was found for the 5'-GBrU pattern. Since the ionization potential of G is lower than that of A, this sequence selectivity has been dubbed a contrathermodynamic one. In the current work, we employ the Marcus and Marcus-Levich-Jortner theory of ET in order to shed light on the observed effect. By using a combination of Density Functional Theory (DFT) and solvation continuum models, we calculated the electronic couplings, reorganization energies, and thermodynamic stimuli for electron transfer which enabled the rates of forward and back ET to be estimated for the two considered sequences. The calculated rates show that the photoreaction could not be efficient if the ET process proceeded within the considered dimers. Only after introducing additional adenines between G and BrU, which accelerates the forward and slows down the back ET, is a significant amount of photodamage expected.