Os mecanismos escudos do DNA para se proteger contra a radiação ultravioleta

quinta-feira, dezembro 09, 2010

How Do DNA Components Resist Damaging UV Exposure?

ScienceDaily (Dec. 9, 2010) — The genetic material of DNA contains shielding mechanisms to protect itself from the exposure to the UV light emitted by the sun. This is of crucial importance, since without photostability -- i.e. without "programmed" defense mechanisms against UV irradiation -- there would be a rapid degradation of DNA and RNA.

The figure shows the special structures of DNA nucleobases, which – after exposure to solar radiation – are responsible for the ultrafast radiationless deactivation to the electronic ground state. (Credit: Felix Plasser, University of Vienna)

As part of a project funded by the Austrian Science Fund (FWF) a group of researchers led by Hans Lischka, Quantum Chemist of the University of Vienna, Austria, could, for the first time, comprehensively unravel these ultra-fast processes of the photostability of the nucleobases. 

The effect of sunlight on our skin not only leads to tanning, but it also initiates processes that can lead to serious health damage. A research team led by Hans Lischka, Professor at the Institute for Theoretical Chemistry, University of Vienna, Austria, investigated the shielding mechanisms that nature has provided to protect itself against such harmful effects. The strategy here is simple, yet highly complex: As soon as the UV light excites the electrons into a higher energy level, ultra-fast decay brings them back to its original state. In this way electronic energy is converted into heat. This process occurs in an incredibly short amount of time, in up to a quadrillionth of a second.

Read more here/Leia mais aqui: Science Daily


Relaxation mechanisms of UV-photoexcited DNA and RNA nucleobases

Mario Barbatti a,b,1, Adélia J. A. Aquino a, Jaroslaw J. Szymczak a, Dana Nachtigallová c, Pavel Hobza c, and Hans Lischka a,c,1

-Author Affiliations

aInstitute for Theoretical Chemistry, University of Vienna, Waehringerstrasse 17, A 1090 Vienna, Austria;
bMax-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, D 45470 Mülheim an der Ruhr, Germany; and
cInstitute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, Flemingovo nam. 2, CZ-16610 Prague 6, Czech Republic.

Edited* by Josef Michl, University of Colorado at Boulder, Boulder, CO, and approved October 15, 2010 (received for review October 6, 2010)


A comprehensive effort in photodynamical ab initio simulations of the ultrafast deactivation pathways for all five nucleobases adenine, guanine, cytosine, thymine, and uracil is reported. These simulations are based on a complete nonadiabatic surface-hopping approach using extended multiconfigurational wave functions. Even though all five nucleobases share the basic internal conversion mechanisms, the calculations show a distinct grouping into purine and pyrimidine bases as concerns the complexity of the photodynamics. The purine bases adenine and guanine represent the most simple photodeactivation mechanism with the dynamics leading along a diabatic ππ* path directly and without barrier to the conical intersection seam with the ground state. In the case of the pyrimidine bases, the dynamics starts off in much flatter regions of the ππ* energy surface due to coupling of several states. This fact prohibits a clear formation of a single reaction path. Thus, the photodynamics of the pyrimidine bases is much richer and includes also nπ* states with varying importance, depending on the actual nucleobase considered. Trapping in local minima may occur and, therefore, the deactivation time to the ground state is also much longer in these cases. Implications of these findings are discussed (i) for identifying structural possibilities where singlet/triplet transitions can occur because of sufficient retention time during the singlet dynamics and (ii) concerning the flexibility of finding other deactivation pathways in substituted pyrimidines serving as candidates for alternative nucleobases.

photodynamical simulation, photostability, ultrafast photodeactivation, nonadiabatic interactions, ab initio multireference methods 


1To whom correspondence may be addressed. E-mail: barbatti@kofo.mpg.deor hans.lischka@univie.ac.at.

Author contributions: H.L. designed research; M.B., A.J.A.A., J.J.S., D.N., and H.L. performed research; M.B., A.J.A.A., J.J.S., D.N., P.H., and H.L. analyzed data; and M.B., A.J.A.A., J.J.S., D.N., P.H., and H.L. wrote the paper.

The authors declare no conflict of interest.

*This Direct Submission article had a prearranged editor.

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