Ultrafast dynamics in gas-phase building blocks of life

In a bottom-up approach to investigate the photo-physical properties of DNA and peptide systems, the individual UV chromophores of these compounds have been studied up to now. The next step consists of studying de-excitation dynamics of larger building blocks, such as nucleosides and aromatic amino acids. The bonding of the sugar ring to the nuclebases, as well as of the glycine moiety to the aromatic chromophores was observed to facilitate and, in some cases, to speed up the relaxation process.

These results are discussed in terms of alternative non-radiative deactivation channels opened up by the added units.<br><br>At higher photon energies or intensities, electron motion and charge transfer can be studied by prompt removal of an electron from the highest occupied molecular orbital. These dynamics are fundamental in understanding the variation of local reactivity and, hence, to predict the possible reaction channels promoted by further photo-absorption.

In aromatic amino acids, the ring structures are strongly involved in internal conversion processes occurring within tens of femtoseconds. Furthermore, fragmentation patterns of nucleosides produced by UV and IR irradiation gave clear evidence of the presence of an ultrafast base/sugar charge transfer in deoxy-thymidine, whereas it was less probable or unlikely in the other nucleosides due to less favourable energetics.<br><br>Much faster dynamics can be initiated by attosecond XUV irradiation, where the superposition of many electronic states produces pure electron motion preceding any nuclear rearrangement.

Cutting-edge laser technology has made possible the observation of ultrafast charge migration in tryptophan by looking at the doubly charged immonium ion channel. The signal modulation with period of 4 fs corresponds to the<br>electron beating between the amino group and the side-chain indole. Oscillation frequency and coherence of this phenomenon are discussed in comparison with the previous experiment in phenylalanine.

These results represent the first ever time-resolved measurements of coherent electron motion in complex biomolecules, which pave the way for steering electron dynamics and controlling molecular reactivity.