Excited States Of DNA Research Articles

Time-resolved cathodoluminescence of DNA triggered by picosecond electron bunches

Despite the tremendous importance of so-called ionizing radiations (X-rays, accelerated electrons and ions) in cancer treatment, most studies on their effects have focused on the ionization process itself, and neglect the excitation events the radiations can induce. Here, we show that the excited states of DNA exposed to accelerated electrons can be studied in the picosecond time domain using a recently developed cathodoluminescence system with high temporal resolution. Our study uses a table-top ultrafast, UV laser-triggered electron gun delivering picosecond electron bunches of keV energy. This scheme makes it possible to directly compare time-resolved cathodoluminescence with photoluminescence measurements. This comparison revealed qualitative differences, as well as quantitative similarities between excited states of DNA upon exposure to electrons or photons.

UV-Induced Proton Transfer between DNA Strands.

UV radiation creates excited states in DNA that lead to mutagenic photoproducts. Photoexcitation of single-stranded DNA can transfer an electron between stacked bases, but the fate of excited states in the double helix has been intensely debated. Here, photoinduced interstrand proton transfer (PT) triggered by intrastrand electron transfer (ET) is detected for the first time by time-resolved vibrational spectroscopy and quantum mechanical calculations. Long-lived excited states are shown to be oppositely charged base pair radical ions. In two of the duplexes, the base pair radical anions are present as tautomers formed by interstrand PT. Charge recombination occurs on the picosecond time scale preventing the accumulation of damaging radicals or mutagenic tautomers.

Base stacking in adenosine dimers revealed by femtosecond transient absorption spectroscopy.

Excitons formed in DNA by UV absorption decay via poorly understood pathways that can culminate in mutagenic photoproducts. In order to gain insight into how base stacking influences UV excited states in DNA, five dinucleosides composed of adenosine or 2'-deoxyadenosine units joined by flexible linkers were studied by femtosecond transient absorption spectroscopy. In aqueous solution, transient absorption signals recorded at pump and probe wavelengths of 267 and 250 nm, respectively, show that UV absorption produces excimer states in all dimers that decay orders of magnitude more slowly than excitations in a single adenine nucleotide. Adding methanol as a cosolvent disrupts π-π stacking of the adenine moieties and causes the excimer states in all five dinucleosides to vanish for a methanol concentration of 80% by volume. These observations confirm that base stacking is an essential requirement for the slow decay channel seen in these and other DNA model compounds. This channel appears to be insensitive to the precise stacking conformation at the instant of photon absorption as long as the bases are cofacially stacked. Notably, circular dichroism (CD) spectra of several of the dinucleosides are weak and monomer-like and lack the exciton coupling that has been emphasized in the past as an indicator of base-stacked structure. For these dimers, the coupled transition dipole moments of the two adenines are proposed to adopt left- and right-handed arrangements upon stacking with roughly equal probability. Although the mechanism behind slow nonradiative decay in DNA is still uncertain, these results show that the signature of these states in transient absorption experiments can be a more reliable diagnostic of base stacking than the occurrence of exciton-coupled CD signals. These observations also draw attention to the important role the backbone plays in producing structures with axial (helical) chirality.

Charge separation and charge delocalization identified in long-living states of photoexcited DNA

Base stacking in DNA is related to long-living excited states whose molecular nature is still under debate. To elucidate the molecular background we study well-defined oligonucleotides with natural bases, which allow selective UV excitation of one single base in the strand. IR probing in the picosecond regime enables us to dissect the contribution of different single bases to the excited state. All investigated oligonucleotides show long-living states on the 100-ps time scale, which are not observable in a mixture of single bases. The fraction of these states is well correlated with the stacking probabilities and reaches values up to 0.4. The long-living states show characteristic absorbance bands that can be assigned to charge-transfer states by comparing them to marker bands of radical cation and anion spectra. The charge separation is directed by the redox potential of the involved bases and thus controlled by the sequence. The spatial dimension of this charge separation was investigated in longer oligonucleotides, where bridging sequences separate the excited base from a sensor base with a characteristic marker band. After excitation we observe a bleach of all involved bases. The contribution of the sensor base is observable even if the bridge is composed of several bases. This result can be explained by a charge delocalization along a well-stacked domain in the strand. The presence of charged radicals in DNA strands after light absorption may cause reactions--oxidative or reductive damage--currently not considered in DNA photochemistry.

Triplet–Triplet Energy Transfer in DNA: A Process that Occurs on the Nanosecond Timescale

No will to wander: Triplet–triplet electronic energy transfer in polyA–polyT DNA sequences was studied by using semiempirical quantum-chemical methods coupled to classical molecular-dynamics simulations. Triplet excited states in DNA were found to be almost completely localized on single nucleobases; the characteristic time for their migration along the A–A and T–T stacks (see picture) was found to be 0.8 and 6.4 ns. Detailed facts of importance to specialist readers are published as ”Supporting Information”. Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

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

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.

Temperature-Dependent Excited State Absorption in DNA and LNA Oligomers Supports an Emerging Model of Excited State Dynamics in DNA

Transient absorption measurements of excited states in DNA and LNA were performed using a femtosecond pump-probe arrangement with excitation at 266 nm and absorption monitored at 400 nm while varying the sample tem- perature between 5 °C and 70 °C. Samples consisted of adenine monophosphate monomer, polyadenine 12-mer in single- stranded form, and polyadenine 12-mer in hybridized form. Excited states decayed in a biphasic manner with short-lived ( 1) and long-lived ( 2) components, while the monomer had only a 'single' short-lived decay time. Temperature increases increased absorption intensities and reduced  1 until they approached those of the monomer at high temperatures (where stacking is minimal). These results suggest that the initial excitation in stacked regions is cooperative and involves several bases and that the number of bases involved is reduced with increasing temperature. In contrast, increasing temperatures had little effect on  2 while absorption intensities decreased, suggesting that very few, perhaps only two, stacked bases are involved and that their number is reduced at higher temperatures. We found no clear evidence of melting point transitions indicating that those excited states probed with our arrangement were not dependent on base pairing. Our results are con- sistent with and strengthen an emerging consensus model of excited state dynamics in DNA wherein a UV photon is ab- sorbed collectively by electronically coupled and thus well-stacked intrachain bases. This collective excitation results in a Frenkel exciton that is delocalized over these bases, and the Frenkel exciton then decays rapidly to a long-lived, lower en- ergy, dark intrachain exciplex.

DENATURATION AND EXCITED STATES OF SHORT DNAs

The statistical mechanics of a finite number of hydrogen bonds in very short DNAs is studied in the Peyrard–Bishop model with an estimate on the significance of the excited states of DNA.

Luminescence of Nucleosides, Nucleotides, and DNA at 77°K

In order to obtain detailed information on the u.v.-induced energy transfer and the excited states in DNA, the fluorescence and phosphorescence of nucleosides, nucleotides, and DNA in 1:1 by volume of ethylene glycol-water at 77°K have been studied. Our main conclusions are: (1) At neutral pH, both the purine derivatives and the pyrimidine derivatives phosphoresce, with the lifetimes of 1.25–2.45 sec and 0.45–0.65 sec, respectively. (2) The lowest triplet state energy of the five bases decreases in the following order: cytosine, guanine, adenine, uracil, thymine. (3) The phosphorescence of DNA is composed of the two components which originate from the neutral thymine and the neutral adenine, but the lowest triplet state of DNA is almost located on the neutral thymine bases (more than 95%). (4) The fluorescence of DNA, which is red-shifted by about 35 mµ from an equimolar mixture of its constituent nucleotides, is understood in terms of the interaction of bases stacked on the same strand in the first excit...

The excited states of DNA.

The excited states of DNA.