Pure and Applied Chemistry

Abstract: Recent experimental and theoretical investigations dealing with model DNA double helices, composed of either adenine–thymine (A–T) or guanine–cytosine (G–C) base pairs, and G quadruplexes shed some light on the excited states populated by photon absorption and their relaxation, energy transfer among bases, and one-photon ionization. These studies revealed that the Franck–Condon excited states of DNA helices cannot be considered as the sum of their monomeric constituents because electronic coupling induces delocalization of the excitation over a few bases. Energy transfer takes place via intraband scattering in less than 100 fs. The fluorescence lifetimes of DNA helices detected by fluorescence upconversion and corresponding mainly to ππ* transitions are longer than that of an equimolar mixture of nucleotides; the only exception was observed for alternating G–C polymers. Moreover, nanosecond flash photolysis experiments showed that organization of bases within single and double helices may lead to a lowering of their ionization potential. Finally, the first determination regarding the time-scale needed for the formation of T dimers, the (6–4) adducts, was determined for the single strand (dT)₂₀.