XIII European Conference on the Spectroscopy of ... - ecsbm 2009

Palermo, August 28 – September 2, 2009 13 th ECSBM −−−− Book of Abstracts
Theoretical search for molecular mechanisms of
photostability of building blocks of life
ANDRZEJ L. SOBOLEWSKI
Institute of Physics, Polish Academy of Sciences, PL-02668 Warsaw, Poland
The DNA and RNA bases absorb strongly in the 200 – 300 nm range, which renders living matter
potentially vulnerable to the UV components of sunlight. Nevertheles, DNA is inherently highly
photostable, that is, the quantum yield of photochemical reaction products is very low.
Apparently, ultrafast photophysical excited-state deactivation-process depopulates the excited
states where destructive reactions can take place. The unstructured absorption spectra of
aminoacids and polypeptides, the generally very low quantum yield of fluorescence (with the
exception of tryptophan containing proteins) indicate similar mechanisms of photostability exist in
these species. This presentation addresses the calculation of reaction paths and the corresponding
energy profiles in excited electronic states of selected DNA bases and their dimers as well as
aminoacids and short peptide chains using ab initio methods. Adenine and guanine-cytosine base
pair are considered as examples of the former systems, while tyrosine-(H2O)2 and Gly-Phe-Ala
peptide represent the latter. For isolated adenine the calculations reveal the reaction pathway
which leads to a conical intersection (CI) with the ground state via out-of-plane deformation of the
hetero-aromatic ring in the lowest excited singlet state [1]. For hydrogen-bonded systems (GC
dimer, Tyr-(H2O)2, and GlyPheAla) another mechanism of deactivation is identified. This involves
electron-transfer followed by single-proton-transfer processes along existing hydrogen-bonds.
These reaction paths lead to CIs with the electronic ground state and provide a pathway for
ultrafast radiationless deactivation [2-4]. These findings suggest that CIs of the excited electronic
states with the electronic ground state play a universal role in the photochemistry of molecular
building blocks of life (amino acids, DNA bases, and their complexes). The conical intersections are
the origin of the exceptional photostability of these compounds, which may have lead to their
selection at the very beginning of the biological evolution [5].
References:
[1] S. Perun, A.L. Sobolewski, W. Domcke, J. Am. Chem. Soc. 2005, 127, 6257
[2] L. Sobolewski, W. Domcke, C. Hättig, Proc. Nat. Acad. Sci. 102 (2005) 17903
[3] L. Sobolewski, D. Shmesh, W. Domcke, J. Phys. Chem. A, 113 (2009) 542
[4] D. Shmesh, A.L. Sobolewski, W. Domcke, J. Am. Chem. Soc., 131 (2009) 1374
[5] C. Sagan, J. Theor. Biol., 39 (1973) 195
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