Biradical radiationless decay channel in adenine and its derivatives

作者：Zgierski Marek Z^*; Patchkovskii Serguei; Lim Edward C

来源：Canadian Journal of Chemistry, 2007, 85(2): 124-134.

DOI：10.1139/V07-006

• adenine
• guanine
• DNA damage
• radiationless decay
• biradical
• ab initio
• coupled clusted

Coupled-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 pi pi* 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 pi* 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 pi pi* 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 pi pi* 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 pi pi* 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.

• 出版日期2007-2