Exploration of the excited-state dynamics of amino acids and peptides by ab initio electronic-structure calculations

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The structure-selective photophysical properties of isolated guanine-cytosine (GC) and adenine-thymine (AT) DNA base pairs will be explored by ab initio electronic-structure calculations. The calculations are intended to guide and support spectroscopic investigations of the lowest-energy conformers of GC and AT by M.S. de Vries and collaborators. The UV spectra of GC provide evidence for a very efficient excited-state deactivation mechanism that is specific for the Watson-Crick tautomer of GC and may be essential for the photostability of DNA (A. Abo-Riziq et al., PNAS 102, 20 (2005)). Preliminary computational evidence hints at the importance of electron-driven inter-base proton-transfer processes for the ultrafast deactivation of the UV absorbing states of GC (A.L. Sobolewski et al., PNAS 102, 17903 (2005)). In this project, we shall perform more comprehensive and more accurate calculations of excited-state potential-energy surfaces, photochemical reaction paths and conical intersections for the lowest-energy tautomers of GC and AT. In addition, the timescales of the relevant primary processes will be estimated via the determination of energy barriers and the simulation of nonadiabatic quantum wave-packet dynamics at conical intersections. Summary of the proposed interaction The spectroscopic investigations will be performed in the research group of M. S. de Vries in Santa Barbara. The computational studies will be performed, in close coordination with the spectroscopic studies, in Garching in the group of W. Domcke. The role of the computations is two-fold: they will (i) guide the experimental studies in the selection of the most interesting systems and the most appropriate spectroscopic techniques and (ii) they will provide support for the interpretation of the experimental signals. Much of the exchange of information will occur by email or file transfer. For the coordination of the research, each of the principal investigators will visit the partner group once per year for one week.