Optical absorption spectra and monomer interaction in polymers. Investigation of exciton coupling in DNA hairpins

We investigate the effect of exciton coupling on the optical absorption spectrum of polymer molecules under conditions of strong inhomogeneous broadening. We demonstrate that the dependence of the maximum in the rescaled absorption spectrum on the number of monomers is determined by the average monomer excitation energies and their resonant coupling and insensitive to the inhomogeneous broadening. Thus the absorption spectrum can be used to determine optical interactions between monomers. The results are applied to the absorption spectra of poly-A poly-T DNA hairpins and used to interpret the dependence of the absorption spectrum on the number of monomers. We also discuss exciton localization in these hairpins.Comment: Submitted to Journal of Chemical Physic

Molecular spectroscopy: Complexity of excited-state dynamics in DNA

Absorption of ultraviolet light by DNA is known to lead to carcinogenic mutations, but the processes between photon absorption and the photochemical reactions are poorly understood. In their study of the excited-stated dynamics of model DNA helices using femtosecond transient absorption spectroscopy1, Crespo-Hernández et al. observe that the picosecond component of the transient signals recorded for the adenine–thymine oligonucleotide (dA)18(dT)18 is close to that for (dA)18, but quite different from that for (dAdT)9(dAdT)9; from this observation, they conclude that excimer formation limits excitation energy to one strand at a time. Here we use time-resolved fluorescence spectroscopy to probe the excited-state dynamics, which reveals the complexity of these systems and indicates that the interpretation of Crespo-Hernández et al. is an oversimplification. We also comment on the pertinence of separating base stacking and base pairing in excited-state dynamics of double helices and question the authors' assignment of the long-lived signal component found for (dA)18(dT)18 to adenine excimers

Fluorescence of the DNA Double Helix (dA)20·(dT)20 Studied by Femtosecond Spectroscopy-Effect of the Duplex Size on the Properties of the Excited States

International audienceThe fluorescence of the DNA double-stranded oligomer (dA)20·(dT)20 is studied at room temperature by fluorescence up-conversion at times shorter than 10 ps. The profile of the up-conversion spectra is similar to that of the steady-state fluorescence spectrum, showing that the majority of the photons are emitted within the probed time scale. At all the probed wavelengths, the fluorescence decays are slower than those of the monomeric chromophores dAMP and TMP. The fluorescence anisotropy decays show strong wavelength dependence. These data allow us to conclude that energy transfer takes place in this double helix and that this process involves exciton states. The spectral and dynamical properties of the oligomer are compared to those of the polymer poly(dA)·poly(dT), composed of about 2000 base pairs, reported previously. The oligomer absorption spectrum is characterized by a smaller hypsochromic shift and weaker hypochromism compared to the polymer. Moreover, the fluorescence decays of (dA)20·(dT)20 are twice as fast as those of poly(dA)·poly(dT), and its fluorescence anisotropy decays more slowly. These differences are the fingerprints of a larger delocalization of the excited states induced by an increase in the size of the duplex

Adenine, deoxyadenosine and deoxyadenosine 5'-monophosphate studied by femtosecond fluorescence upconversion spectroscopy

International audienceAqueous solutions of adenine (A), deoxyadenosine (dA) and deoxyadenosine 5′-monophosphate (dAMP) were studied in room temperature by femtosecond fluorescence upconversion. The fluorescence decays cannot be described by single exponentials. They consist of an ultrafast component (230 fs for A, <100 fs for dA and dAMP) and a slower one (8 ps for A, 0.5 ps for dA and dAMP). The slow component constitutes 95% of the total fluorescence (time-integrated) for the base while only 24% for the nucleoside or the nucleotide. The initial fluorescence anisotropy is 0.30±0.03 for A, 0.25±0.05 for dA and dAMP. The anisotropy of the A fluorescence partially decays during its lifetime due to rotational diffusion

Fluorescence of the DNA double helices (dAdT)n·(dAdT)n studied by femtosecond spectroscopy

International audiencePolymeric and oligomeric DNA helices, poly(dAdT)·poly(dAdT) and (dAdT)10·(dAdT)10, composed of 200–400 and 20 adenine–thymine base pairs, respectively, are studied by fluorescence upconversion. Fluorescence decays, anisotropy decays and time-resolved spectra, obtained for this alternating base sequence, are compared with those determined previously for the homopolymeric sequence (dA)n·(dT)n. It is shown that identical fluorescence decays may correspond to quite different anisotropy decays and vice versa, both varying with the emission wavelength, the base sequence and the duplex size. Our observations cannot be explained in terms of monomer and excimer emission exclusively, as concluded in the past on the basis of steady-state measurements. Excitons also contribute to the fluorescence. These are rapidly trapped by excimers, characterized by long-lived weak emission