title: High-Energy Long-Lived Mixed Frenkel-Charge-Transfer Excitons: From Double Stranded (AT)n to Natural DNA.
abstract: The electronic excited states populated upon absorption of UV photons by DNA are extensively studied in relation to the UV-induced damage to the genetic code. Here, we report a new unexpected relaxation pathway in adenine-thymine double-stranded structures (AT)n . Fluorescence measurements on (AT)n hairpins (six and ten base pairs) and duplexes (20 and 2000 base pairs) reveal the existence of an emission band peaking at approximately 320 nm and decaying on the nanosecond time scale. Time-dependent (TD)-DFT calculations, performed for two base pairs and exploring various relaxation pathways, allow the assignment of this emission band to excited states resulting from mixing between Frenkel excitons and adenine-to-thymine charge-transfer states. Emission from such high-energy long-lived mixed (HELM) states is in agreement with their fluorescence anisotropy (0.03), which is lower than that expected for π-π* states (≥0.1). An increase in the size of the system quenches π-π* fluorescence while enhancing HELM fluorescence. The latter process varies linearly with the hypochromism of the absorption spectra, both depending on the coupling between π-π* and charge-transfer states. Subsequently, we identify the common features between the HELM states of (AT)n structures with those reported previously for alternating (GC)n : high emission energy, low fluorescence anisotropy, nanosecond lifetimes, and sensitivity to conformational disorder. These features are also detected for calf thymus DNA in which HELM states could evolve toward reactive π-π* states, giving rise to delayed fluorescence.
authors.affiliation: LIDYL, CEA, CNRS, Université Paris Saclay, 91191, Gif-sur-Yvette, France.
authors.forename: Ignacio
authors.initials: I
authors.lastname: Vayá
authors.affiliation: LIDYL, CEA, CNRS, Université Paris Saclay, 91191, Gif-sur-Yvette, France.
authors.forename: Johanna
authors.initials: J
authors.lastname: Brazard
authors.affiliation: LIDYL, CEA, CNRS, Université Paris Saclay, 91191, Gif-sur-Yvette, France.|Institut für Physikalische und Theoretische Chemie, Goethe-Universität, Max-von-Laue-Str. 7, 60438, Frankfurt am Main, Germany.
authors.forename: Miquel
authors.initials: M
authors.lastname: Huix-Rotllant
authors.affiliation: Department of Chemistry, Northwestern University, Evanston, Illinois, 60208, USA.
authors.forename: Arun K
authors.initials: AK
authors.lastname: Thazhathveetil
authors.affiliation: Department of Chemistry, Northwestern University, Evanston, Illinois, 60208, USA. fdl@northwestern.edu.
authors.forename: Frederick D
authors.initials: FD
authors.lastname: Lewis
authors.affiliation: LIDYL, CEA, CNRS, Université Paris Saclay, 91191, Gif-sur-Yvette, France.
authors.forename: Thomas
authors.initials: T
authors.lastname: Gustavsson
authors.affiliation: Institut für Physikalische und Theoretische Chemie, Goethe-Universität, Max-von-Laue-Str. 7, 60438, Frankfurt am Main, Germany.
authors.forename: Irene
authors.initials: I
authors.lastname: Burghardt
authors.affiliation: Istituto Biostrutture e Bioimmagini-Consiglio Nazionale delle Ricerche, Via mezzocannone 16, 80136, Napoli, Italy. robimp@unina.it.
authors.forename: Roberto
authors.initials: R
authors.lastname: Improta
authors.affiliation: LIDYL, CEA, CNRS, Université Paris Saclay, 91191, Gif-sur-Yvette, France. Dimitra.markovitsi@cea.fr.
authors.forename: Dimitra
authors.initials: D
authors.lastname: Markovitsi
mesh_terms: D000225:Adenine / Q000737:chemistry*; D000818:Animals; D002417:Cattle; D003596:Cytosine / Q000737:chemistry*; D004247:DNA / Q000737:chemistry* / Q000378:metabolism; D004735:Energy Transfer; D009841:Oligonucleotides / Q000138:chemical synthesis* / Q000737:chemistry; D011789:Quantum Theory; D013050:Spectrometry, Fluorescence; D013941:Thymine / Q000737:chemistry*; D014466:Ultraviolet Rays
chemical_list: D009841:Oligonucleotides; D003596:Cytosine; D004247:DNA; C427203:calf thymus DNA; D000225:Adenine; D013941:Thymine
keywords: DNA; excitons; fluorescence spectroscopy; hypochromism; quantum chemistry