DNA Excited-State Dynamics: From Single Bases to the Double Helix

Abstract

Ultraviolet light is strongly absorbed by DNA, producing excited electronic states that sometimes initiate damaging photochemical reactions. Fully mapping the reactive and nonreactive decay pathways available to excited electronic states in DNA is a decades-old quest. Progress toward this goal has accelerated rapidly in recent years, in large measure because of ultrafast laser experiments. Here we review recent discoveries and controversies concerning the nature and dynamics of excited states in DNA model systems in solution. Nonradiative decay by single, solvated nucleotides occurs primarily on the subpicosecond timescale. Surprisingly, excess electronic energy relaxes one or two orders of magnitude more slowly in DNA oligo- and polynucleotides. Highly efficient nonradiative decay pathways guarantee that most excited states do not lead to deleterious reactions but instead relax back to the electronic ground state. Understanding how the spatial organization of the bases controls the relaxation of excess electronic energy in the double helix and in alternative structures is currently one of the most exciting challenges in the field.

Mapping the Ultrafast Dynamics of Adenine onto Its Nucleotide and Oligonucleotides by Time-Resolved Photoelectron Imaging

“…It has been suggested that differences in In solution, additional long-lived dynamics (10s -100s ps) observed in transient absorption spectra of d(A) n -(n ≥ 2) have been assigned to the formation of excimer states that are delocalised over two (or more) adjacent π-stacked bases. 2,3,[38][39][40] In our data, no evidence for the formation of long-lived states, excimer or otherwise has been observed. There may be several reasons for the lack of excimer dynamics observed.…”

“…1 Despite the efficient UV absorption, mediated by the optically bright 1 ππ* states localised on the four DNA nucleobases, the photodamage quantum yield in DNA is low (<1%). 2,3 This photostability is governed by the non-radiative decay mechanisms that enable the nucleobases to assimilate and dispose of the potentially harmful electronic energy in a non-destructive fashion.Gaining a molecular level understanding of these processes has been a long-standing goal, not only because of its role in radiation damage of DNA, but also to assess why nature has evolved using such a select number of molecular building blocks to define the genetic code. 4 Much of the experimental effort has been devoted to the fate of adenine (Ade) following excitation to its 1 ππ* states.…”

On the Participation of Photoinduced N–H Bond Fission in Aqueous Adenine at 266 and 220 nm: A Combined Ultrafast Transient Electronic and Vibrational Absorption Spectroscopy Study

“…[4][5][6] As such, the DNA and RNA nucleobases are classed as photostable species. From a photochemistry perspective, photostability is often characterized by short (<1 ps) electronically excited state lifetimes, low fluorescence quantum yields (Φf < 10 -4 ) and a strong resistance to the formation of undesired and potentially harmful photoproducts.…”

Solvent-Induced Shifts in Electronic Spectra of Uracil

“…1,2 Solvent effects on the photochemistry of nucleotide bases have been widely studied. In particular, the high sensitivity of the excited states of uracil to a solvent has made it a popular system for studying photochemistry.…”