Biradical radiationless decay channel in adenine and its derivatives

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Journal titleCanadian Journal of Chemistry
Pages124134; # of pages: 11
Subjectadenine, guanine, DNA damage, radiationless decay, biradical, ab initio, coupled clusted
AbstractCoupled-cluster calculations of increasing accuracy (approximate doubles: CC2; doubles: EOM-CCSD; connected triples: CR-EOM-CCSD(T)) for CIS-optimized potential energy profiles of adenine and its derivatives indicate that the ultrafast internal conversion of the optically excited π π* state occurs through a state switch to a biradical state, which intersects the ground state at a lower energy. The electronic nature of the biradical state is defined by an electronic configuration in which one unpaired electron occupies a π* orbital confined to the five-membered ring. The second unpaired electron is localized very strongly on a p-type C2 atomic orbital of the six-membered ring. The biradical state minimum has a strongly puckered six-membered ring and a C2–H bond, which is twisted nearly perpendicular to the average ring plane. Consistent with the biradical-mediated internal conversion, the π π* state lifetime is extremely short in adenine and 9-methyladenine, which have barrierless crossing to the biradical state. The lifetime is slightly longer in N,N-dimethyladenine, which has a small barrier for the state switch. In 2-aminopurine the biradical state is found above the π π* state, preventing the biradical state switch and dramatically increasing the lifetime. These results, combined with an earlier work on pyrimidine bases, strongly suggest the importance of a direct decay of the doorway π π* state via a biradical state switch in the photophysics of DNA, even though the nature of the biradical state is somewhat different in purines and pyrimidines.
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PublisherNRC Research Press
AffiliationNational Research Council Canada; NRC Steacie Institute for Molecular Sciences
Peer reviewedYes
NPARC number12328624
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Record identifier91cb9122-c111-42dc-abb4-eb2a1fbb23cf
Record created2009-09-10
Record modified2017-07-10
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