Excited states and energy transfer among DNA bases in double helices

Abstract

Abstract:The study of excited states and energy transfer in DNA double helices has recently gained new interest connected to the development of computational techniques and that of femtosecond spectroscopy. The present article points out contentious questions regarding the nature of the excited states and the occurrence of energy transfer and shows how they are currently approached. Using as example the polymer poly(dA).poly(dT), composed of about 2000 adenine-thymine pairs, a model is proposed on the basis of time-resolved measurements (fluorescence decays, fluorescence anisotropy decays and fluorescence spectra, obtained with femtosecond resolution), associated to steady-state spectra. According to this qualitative model, excitation at 267 nm populates excited states that are delocalized over a few bases (excitons). Ultrafast internal conversion directs the excited state population to the lower part of the exciton band giving rise to fluorescence. Questions needing further investigations, both theoretical and experimental, are underlined with particular emphasis on delicate points related to the complexity and the plasticity of these systems.

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

“…After femtosecond excitation at 267 nm, we detected fluorescence at various wavelengths, over a large time-domain by means of two different techniques, fluorescence upconversion and time-correlated single photon counting. 11,12 These time-resolved data, associated with the steady-state absorption and fluorescence spectra, lead to the conclusion that the Franck-Condon excited states are delocalized over several bases, in line with results from circular dichroïsm performed on duplexes containing the same base sequence. 13,14 The initial low value and subsequent decrease of fluorescence anisotropy observed for poly(dA).poly(dT), on the subpicosecond time-scale, 11 when molecular motions 4 18/02/2010 are hindered, proved that energy transfer takes place among the bases in less than 100 fs via intraband scattering.…”

“…Laser pulses populate a large number of more or less delocalized excited states. The "large number" is related to both conformation disorder and to homogenous broadening of monomer 12 18/02/2010 electronic transitions. 24 Emission can be viewed as ranging between two limiting cases ( Figure 8).…”

“…18 Such an increased structural disorder may limit the degree of delocalization. Buchvarov et al, 17 found indeed a weak difference between (dA) 12 . (dT) 12 and (dA) 18 .…”

“…Buchvarov et al, 17 found indeed a weak difference between (dA) 12 . (dT) 12 and (dA) 18 . (dT) 18 .…”

“…12 However, in a quantitative way, quite noticeable differences are found between the two systems. The spectral and dynamical properties of the excited states, which in principle are determined by the local environment represented by a few bases, are in fact affected significantly by factors extended to much longer distances.…”