Ultrafast excited state deactivation and energy transfer in guanine-cytosine DNA double helices

International audienceThe stability of DNA components with respect to UV radiation is considered to be a prerequisite for the development of the genetic code. But it is also known that UV light absorbed by DNA bases may damage the double helix and lead to carcinogenic mutations.1 The interplay between stability and photodamage depends on the way that the energy of a UV photon is distributed among the electronic excited states of the double helix before it is eliminated as heat. Ultrafast dissipation of the excitation energy is indeed a common property of all the monomeric DNA building blocks: the major part of the excited state population of nucleosides and nucleotides in aqueous solution lives for less than one picosecond.2,3 When applied to double helices, composed exclusively of adenine-thymine base pairs (A-T duplexes, both homopolymeric and alternating), femtosecond spectroscopy reveals a different picture: organization of the bases within duplexes causes an overall lengthening of the excited state lifetimes.4-9 This is due to the emergence of new excited states, shared between at least two bases. The existence of delocalized excited states allows ultrafast energy transfer to occur5,8-10 by-passing the prerequisites of Förster transfer which are not fulfilled in the case of DNA bases

Excited-state dynamics of dGMP measured by steady-state and femtosecond fluorescence spectroscopy.

International audienceThe room-temperature fluorescence of 2'-deoxyguanosine 5'-monophosphate (dGMP) in aqueous solution is studied by steady-state and time-resolved fluorescence spectroscopy. The steady-state fluorescence spectrum of dGMP shows one band centered at 334 nm but has an extraordinary long red tail, extending beyond 700 nm. Both the fluorescence quantum yield and the relative weight of the 334 nm peak increase with the excitation wavelength. The initial fluorescence anisotropy after excitation at 267 nm is lower than 0.2 for all emission wavelengths, indicating an ultrafast S(2) --> S(1) internal conversion. The fluorescence decays depend strongly on the emission wavelength, getting longer with the wavelength. A rise time of 100-150 fs was observed for wavelengths longer than 450 nm, in accordance with a gradual red shift of the time-resolved spectra. The results are discussed in terms of a relaxation occurring mainly on the lowest excited (1)pi pi*-state surface toward a conical intersection with the ground state, in line with recent theoretical predictions. Our results show that the excited-state population undergoes a substantial "spreading out" before reaching the CI, explaining the complex dynamics observed

Excited States and Energy Transfer in G-Quadruplexes

International audienceDNA nanostructures formed by association of four oligonucleotides d(TGGGGT) (TG4T quadruplexes) are studied by steady-state and time-resolved optical spectroscopy with femtosecond resolution using fluorescence upconversion. A comparison between single-stranded and four-stranded structures and the corresponding stoichiometric mixture of noninteracting nucleotides shows how horizontal and vertical organization affects the properties of the excited states. Emission from guanine excimers is observed only for single strands, where conformational motions favor their formation. Quadruplex fluorescence arises from a multitude of excited states generated via electronic coupling between guanines; the average fluorescence lifetime is longer and the fluorescence quantum yield higher compared to those of noninteracting nucleotides. The fluorescence anisotropy recorded on the subpicosecond time scale, where molecular motions are hindered, reveals that energy transfer takes place among the bases composing the nanostructure. These results are in line with the conclusions drawn from similar studies on model DNA duplexes

The local structure in C4mimPF6/acetonitrile mixture: The charge distribution effect

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Probing Structural patterns of ion association and solvation dynamics of imidazolium ionic liquids with acetonitrile, propylene carbonate and gamma-Butyrolactone: A molecular dynamics analysis

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