Thursday 26 July 2012

UV absorption in DNA

There is another paper by us, which is just in the final stages of being published: "The UV absorption spectrum of alternating DNA duplexes - analysis of excitonic and charge transfer interactions". The idea was to simulate the absorption spectra of alternating DNA duplexes and analyze the excited states according to charge transfer character and excitonic delocalization.

The method we used was ADC(2), which has the advantage of not being as computationally expensive as CASPT2 while not being TDDFT... By now also the TDDFT studies have rather convergent results and raised the charge transfer states to the energy of the bright states. But it took a lot of reparametrization and "alchemy" to get there in my opinion. But then I am of course not an expert when it comes to TDDFT ...

The environment was simulated by QM/MM electrostatic embedding. Advantage: atomistic description of the environment, disadvantage: neglect of all non-electrostatic terms. The spectra decomposition was performed according to the analysis method described in this post.

The main results are
  • the states are rather localized
  • charge transfer states are at higher energies than the bright states in the Franck-Condon region
  • there is a significant coupling between charge transfer and locally excited (Frenkel) states
  • spectral broadening is mainly caused by intramolecular vibrations

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