55 research outputs found
Electronic excited states of guanine-cytosine hairpins and duplexes studied by fluorescence spectroscopy
[EN] Guanine-cytosine hairpins, containing a hexaethylene glycol bridge, are studied by steady-state fluorescence spectroscopy and time-correlated single photon counting; their properties are compared to those of duplexes with the same sequence. It is shown that, both in hairpins and in duplexes, base pairing induces quenching of the pi pi(star) fluorescence, the quantum yield decreasing by at least two orders of magnitude. When the size of the systems increases from two to ten base pairs, a fluorescent component decaying on the nanosecond time-scale appears at energy higher than that stemming from the bright states of non-interacting mono-nucleotides (ca. 330 nm). For ten base pairs, this new fluorescence forms a well-defined band peaking at 305 nm. Its intensity is about 20% higher for the hairpin compared to the duplex. Its position (red-shifted by 1600 cm(-1)) and width (broader by 1800 cm(-1) FWHM) differ from those observed for large duplexes containing 1000 base pairs, suggesting the involvement of electronic coupling. Fluorescence anisotropy reveals that the excited states responsible for high energy emission are not populated directly upon photon absorption but are reached during a relaxation process. They are assigned to charge transfer states. According to the emerging picture, the amplitude of conformational motions determines whether instantaneous deactivation to the ground state or emission from charge transfer states will take place, while pi pi(star) fluorescence is associated to imperfect base-pairing.The French Agency for Research (ANR-10-BLAN-0809-01), the Conselleria de Educacion-Generalitat Valenciana (VALi+D
program to I.V., N° 2010033) and the US Department of Energy, Office of Basic Energy Sciences (grant no. DE-FG02-96ER14604 to F.D.L.) are acknowledged for financial support.Brazard, J.; Thazhathveetil, A.; Vayá Pérez, I.; Lewis, F.; Gustavsson, T.; Markovitsi, D. (2013). Electronic excited states of guanine-cytosine hairpins and duplexes studied by fluorescence spectroscopy. Photochemical & Photobiological Sciences Photochemical and Photobiological Sciences. 12(8):1453-1459. https://doi.org/10.1039/c3pp50088hS14531459128Bouvier, B., Dognon, J.-P., Lavery, R., Markovitsi, D., Millié, P., Onidas, D., & Zakrzewska, K. (2003). Influence of Conformational Dynamics on the Exciton States of DNA Oligomers. The Journal of Physical Chemistry B, 107(48), 13512-13522. doi:10.1021/jp036164uMezzina, E., Mariani, P., Itri, R., Masiero, S., Pieraccini, S., Spada, G. P., … Gottarelli, G. (2001). 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DONSON and FANCM associate with different replisomes distinguished by replication timing and chromatin domain
Eukaryotic replisomes are multiprotein complexes. Here the authors reveal two distinct stressed replisomes, associated with DONSON and FANCM, displaying a bias in replication timing and chromatin domain
Hole transport in DNA hairpins via base mismatches and strand crossings: Efficiency and dynamics
Photoselective DNA Hairpin Spin Switches
DNA hairpins having both a tethered
anthraquinone (Aq) end-capping
group and a perylenediimide (PDI) base surrogate were synthesized,
wherein Aq and PDI are each separated from a G-C base pair hole trap
by A-T and I-C base pairs (G = guanine, A = adenine, T= thymine, C
= cytosine, I = inosine). Selective photoexcitation of PDI at 532
nm generates a singlet radical ion pair (RP), <sup>1</sup>(G<sup>+•</sup>-PDI<sup>–•</sup>), while selective photoexcitation
of Aq at 355 nm generates the corresponding triplet RP, <sup>3</sup>(G<sup>+•</sup>-Aq<sup>–•</sup>). Subsequent
radical pair intersystem crossing within these spin-correlated RPs
leads to mixed spin states that exhibit spin-polarized, time-resolved
EPR spectra in which the singlet- and triplet-initiated RPs have opposite
phases. These results demonstrate that a carefully designed DNA hairpin
can serve as a photodriven molecular spin switch based on wavelength-selective
formation of the singlet or triplet RP without significant competition
from undesired energy transfer processes
Relaxation des états excités de guanine‐cytosine alternés en double brin étudiée par spectroscopie de fluorescence résolue en temps
Psoralen Conjugates for Visualization of Genomic Interstrand Cross-Links Localized by Laser Photoactivation
Ultrafast Excited-State Dynamics in Hexaethyleneglycol-Linked DNA Homoduplexes Made of A·T Base Pairs
Double-stranded DNA conjugates with
the sequence (dA)<sub>10</sub>·(dT)<sub>10</sub> and hexaethylene
glycol linkers at one end
(hairpin) or both ends (dumbbell) were studied in buffer solution
by deep UV femtosecond transient absorption spectroscopy. These covalently
constrained duplexes have greatly enhanced thermal stability compared
to A·T duplex oligonucleotides that lack linkers. The conjugates
eliminate the slipped-strand and end-frayed structures that form readily
in unlinked (dA)<sub><i>n</i></sub>·(dT)<sub><i>n</i></sub> sequences, allowing the excited-state dynamics of
stacked A·T base pairs to be observed without interference from
structures with stacking or pairing defects. Transient absorption
signals show that subpicosecond internal conversion to the electronic
ground state takes place in addition to the formation of long-lived
excited states having lifetimes of approximately 70 ps. Watson–Crick
base-pairing slows the rate of vibrational cooling compared to monomeric
bases or single-stranded DNA, possibly by reducing the total number
of solute–solvent hydrogen bonds. Long-lived excited states
in intact A·T base pairs decay several times more quickly than
long-lived excited states observed in single-stranded (dA)<sub><i>n</i></sub> sequences. These results show that base-pairing
can measurably affect nonradiative decay pathways in A·T duplexes
The DNA Translocase FANCM/MHF Promotes Replication Traverse of DNA Interstrand Crosslinks
The replicative machinery encounters many impediments, some of which can be overcome by lesion bypass or replication restart pathways, leaving repair for a later time. However, interstrand crosslinks (ICLs), which preclude DNA unwinding, are considered absolute blocks to replication. Current models suggest that fork collisions, either from one or both sides of an ICL, initiate repair processes required for resumption of replication. To test these proposals, we developed a single molecule technique for visualizing encounters of replication forks with ICLs, as they occur in living cells. Surprisingly, the most frequent patterns were consistent with replication traverse of an ICL, without lesion repair. The traverse frequency was strongly reduced by inactivation of the translocase and DNA binding activities of the FANCM/MHF complex. The results indicate that translocase-based mechanisms enable DNA synthesis to continue past ICLs, and that these lesions are not always absolute blocks to replication
Effect of Mg<sup>2+</sup> Cations on the Dynamics and Efficiency of Hole Transport in DNA
The effect of Mg<sup>2+</sup> cations
on the electronic spectra
and dynamics and efficiency of hole transport has been determined
by means of femtosecond time-resolved transient absorption spectroscopy
for DNA hairpins possessing stilbene electron acceptor and donor chromophores.
The results are compared with those obtained previously for the same
hairpins in the presence of Na<sup>+</sup> cations and for one hairpin
with no added salt. Quantum yields and rate constants for charge separation
are smaller in the presence of Mg<sup>2+</sup> than Na<sup>+</sup>, the largest differences being observed for the hairpins with the
largest number of base pairs. Slower charge separation is attributed
to minor groove binding by Mg<sup>2+</sup>, which results in a stiffer
duplex structure rather than a change in ground state geometry. Reduction
in the Na<sup>+</sup> concentration has little effect on either the
dynamics or efficiency of hole transport
Dynamics and Efficiency of Hole Transport in LNA:DNA Hybrid Diblock Oligomers
We report here the effect of replacing one or both of
the purine
or pyrimidine blocks of a diblock stilbene donor–acceptor capped
hairpin with locked nucleic acid (LNA) bases on the dynamics and efficiency
of hole transport. The structures of the DNA and LNA:DNA hybrids are
tentatively assigned to B- or A-type structures on the basis of their
circular dichroism spectra. Replacing the bases in either the A-block
or the G-block of the diblock DNA hairpin with LNA bases results in
a modest decrease in the base-to-base hopping rate constant and quantum
yield for charge separation. Somewhat larger decreases are observed
when all of the purine or pyrimidine bases are replaced by LNA bases
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