High-Energy Long-Lived Emitting Mixed Excitons in Homopolymeric Adenine-Thymine DNA Duplexes

Abstract

The publication deals with polymeric pA●pT and oligomeric A20●T20 DNA duplexes whose fluorescence is studied by time-correlated single photon counting. It is shown that their emission on the nanosecond timescale is largely dominated by high-energy components peaking at a wavelength shorter than 305 nm. Because of their anisotropy (0.02) and their sensitivity to base stacking, modulated by the duplex size and the ionic strength of the solution, these components are attributed to mixed ππ*/charge transfer excitons. As high-energy long-lived excited states may be responsible for photochemical reactions, their identification via theoretical studies is an important challenge.

The Ubiquity of High-Energy Nanosecond Fluorescence in DNA Duplexes

“…For all the studied duplexes, either with repetitive ,, or disordered base sequence and, most importantly, for genomic DNA, , a ns component was detected on the blue side (305/310 nm) of the spectrum. Its relative contribution to the total number of emitted photons at these wavelengths is far from negligible, varying from ∼30% to ∼60%.…”

“…In parallel, time-resolved fluorescence experiments performed on longer time-scales revealed unexpected nanosecond components at wavelengths shorter than the monomer fluorescence. − Hereafter, we denote them as HENE, for high-energy nanosecond emission, as opposed to LENE, for low-energy nanosecond emission. A short description of HENE was reported recently in our account of intrinsic DNA fluorescence .…”

“…Effect of the duplex size on the normalized fluorescence decays recorded at 305 nm for double-stranded structures in phosphate buffer: (a) (dGC) n ·(dGC) n hairpins excited at 255 nm ( n = 6, light green; n = 10, dark green); (b) (dA) n ·(dT) n duplexes excited at 285 nm ( n = 20, cyan; n ≈ 1000, blue) …”

“…Vertical gray bars correspond to the total number of photons emitted at a given wavelength while blue and red bars indicate the part of these photons emitted after 1 ns, as determined from the TCSCP decays. (a) (dAT) n ·(dAT) n ; n ≈ 2000, (b) (dA) n ·(dT) n ; n ≈ 1000, (c) (dG) n ·(dC) n ; n ≈ 1200, and (d) calf thymus DNA . The spectra of the corresponding stoichiometric mixtures of mononucleotides are shown in gray.…”

“…A short description of HENE was reported recently in our account of intrinsic DNA fluorescence. 6 In the meantime, new studies 5,21,22 rendered the assessment of the excited states responsible for this intriguing emission much more complex than that proposed by the first computational approaches. 19,20 Therefore, we have decided to revisit the current state of knowledge of this poorly understood, yet ubiquitous, fluorescence emission in the present Perspective.…”