Résumé :
Les brins d’ADN riches en guanine, comme ceux présents à l’extrémité des chromosomes humains, sont capables de s’associer entre eux pour former des structures G-quadruplexes, résultant de l’association de quatre guanines. Ces structures sont actuellement l’objet d’un intérêt particulier pour le développement de nouvelles thérapies anti-cancéreuses et des applications potentielles pour l’électronique moléculaire. Il n’existe cependant que très peu d’études de leurs propriétés photophysiques. L’objectif de ce travail de thèse est d’étudier l’influence de la structure des G-quadruplexes (taille, nature de cations métalliques dans la cavité centrale, topologie) sur le caractère collectif des états excités (excitons), le transfert d’énergie, ainsi que la relaxation des états pp* vers des états à transfert de charge. Les études ont été menées à l’aide de la spectroscopie de fluorescence résolue en temps sur une gamme temporelle allant de la centaine de femtosecondes à la centaine de nanosecondes.
Mots-clés : G-quadruplexes – Fluorescence résolue en temps – Transfert d’énergie – États ππ* – États à transfert de charge
Guanine self-assembled structures studied by time-resolved optical spectroscopy
Abstract:
Guanine rich DNA strands have the ability to form four-stranded structures (G-quadruplexes). Their repetitive unit is the G-quartet (tetrad) where each base is connected with two others via four hydrogen bonds. These structures have a crucial role in biological aspect, as targets for anti-cancer therapies, and have great potential for applications in nanotechnology. We studied the electronic excited states of G-quadruplexes using two different techniques, fluorescence up-conversion (FU) and time-correlated single photon counting (TCSPC) , which allow probing the emissive states over six decades of time (from hundred femtoseconds to hundreds of nanoseconds). At first, we examined the effect of the size of tetramolecular G-quadruplexes on their photophysical properties. We have found that the collective behavior of Franck-Condon excited states is enhanced when the number of tetrads increases. For all systems studied, the anisotropy of the G-quadruplex, on the time scale of hundreds of femtoseconds, is lower than that of non-interacting mono-nucleotides in solution. This decrease in anisotropy is associated with an ultrafast energy transfer process between the bases. Then we demonstrated that the metal cations located in the central cavity of quadruplexes also affect their photophysical properties. In the presence of K+, emission arises mainly from delocalized ππ* states (excitons), whereas in the presence of Na+, it is dominated by the contribution of charge transfer excited states. Finally, we studied the effect of conformation, comparing the properties of tetramolecular G-quadruplexes with those formed by folding a single strand (intramolecular G-quadruplexes). We have shown that the conformation of the nano-structures influences the properties of the excited Franck-Condon states as emissive states as well. These effects are attributed to different geometric arrangement of G-tetrads in tetramolecular and intramolecular quadruplexes.
Keywords: G-quadruplexes – Time-resolved fluorescence – Energy transfer – .ππ* states – Charge transfer state
Groupe Biomolécules Excitées