G-Qadruplexes from Human Telomeric DNA: How Many Conformations in PEG Containing Solutions?

G-quadruplex structures are an attractive target for the development of anticancer drugs as their formation in human telomere induces a DNA damage response followed by apoptosis in cancer cells. However, the development of new anticancer drugs by means of structural-based drug design, is hampered by a lack of an accurate information on the exact G-quadruplex conformation adopted by the human telomeric DNA under physiological condition. Several groups reported that in a molecular crowded, cell-like environment, simulated by polyethylene glycol (PEG), the human telomeric DNA adopts the parallel G-quadruplex conformation. These studies have suggested that 40% (w/v) PEG concentration induces complete structural conversion from the other known human telomeric G-quadruplex conformations to the parallel G-quadruplex, thus simplifying the high structural polymorphism existing in absence of PEG. In this study, we demonstrate that the structural conversion to the parallel G-quadruplex is not a complete reaction at physiological temperature. We report a complete kinetic and thermodynamic characterization of the conformational transitions involving the (TTAGGG)4TT and (TTAGGG)8TT human telomeric DNA sequences in K+ solution containing PEG. Our data show that the hybrid-type and parallel conformations coexist at equilibrium in presence of PEG at the physiological temperature and the degree of the quadruplex interconversion depends on the PEG molecular weight. Further, we find that telomeric DNA folds in the parallel quadruplex in seconds time scale, a much larger time scale than the one reported for the hybrid quadruplex folding (~ms). The whole of our data allow us to predict the relative amount of each G-quadruplex conformation as function of temperature and time. The effect of other crowding agent like Ficoll 400 and glycerol on the quadruplex interconversion has been also explored.