Goal:
We aim for a detailed understanding of photochemical and photophysical processes in biological molecules. Following a reductionist approach, we focus on isolated chromophores and molecular clusters which are readily characterized by experiment and well described by high-level ab initio theory. Micro-solvated clusters mimic solvation and environmental effects and connect our detailed understanding of isolated molecules with the complex structures relevant to biology. By investigating of increasingly complex clusters, we strive for a "bottom up" description of realistic biological systems.

Bottom-up approach to the photochemistry of DNA: Characterization of isolated bases and model base-pairs is followed by investigations of base pairs and solvated bases.

Method:
Femtosecond time-resolved ionization spectroscopy
Molecular clusters are formed in a pulsed supersonic jet expansion and photoexcited by a femtosecond laser pulse (pump). After a variable time delay Dt, the molecules and clusters are ionized by a second femtosecond pulse (probe). For time-resolved mass spectra (TRMS), the ion masses are analyzed in a Wiley-McLaren time-of-flight mass spectrometer. The number of detected ions is directly proportional to the excited state population and allow the investigation of population dynamics in the excited state. For time-resolved photoelectron spectra (TRPES), the energy of the emitted electrons is measured in an electron spectrometer. This spectroscopic information can allow the unambiguous assignment of electronic and nuclear properties along the reaction/relaxation pathway. To distinguish the electronic spectra of different clusters in our molecular beam, we detect both, the electron and the ion, of single ionization events. Such a femtosecond electron-ion coincidence experiment (FEICO) must operate with very low ionization rates (~10^-2) to avoid double ionization events and false coincidences.

Time-resolved ionization spectroscopy: Molecular clusters are formed in a supersonic jet expansion behind a pulsed valve. The skimmed beam is ionized by fs pump and probe pulses and electrons/ions are detected in time-of-flight spectrometers.

Hydrogen transfer in Aminopyridine dimer, a model DNA base pair

Dynamics and structure of aminopyridine clusters: Excited states in the monomer and larger clusters decay in nanoseconds, only the dimer decays in 65 ps. This fast process was assigned to an electron-proton transfer process and correlated with a near-planar H-bound structure resembling a Watson-Crick base pair.
[Science 306, 1765 (2004); J.Am.Chem.Soc. 128, 15652 (2006)]

Excited state relaxation in DNA base pairs

Excited state dynamics in adenine and thymine base pairs: Fast relaxation of the UV-absorbing pi-pi* states in DNA base-pairs may protect the genetic code from photochemical damage. We found similar relaxation properties for monomers and dimers and no special relaxation pathway for the A-T base pair. Our observations indicate an intra-monomer relaxation mechanism in the base pairs.
[J.Am.Chem.Soc. 127, 1782 (2005); ChemPhysChem 8, 751 (2007); ChemPhys 347, 376 (2008)]

Quenching of excited state populations in water clusters

Role of pi-sigma * state in adenine-(H2O) clusters: Ab initio calculations show a stabilization of 0.14?0.36 eV for the pi-sigma * state in isomers of Ad(H2O)1 and Ad(H2O)3 and this state might play an important role in the excited state relaxation of partially or fully solvated adenine. Experimental results concur: The strong n-pi* state signal (t2) in the monomer is quenched in water clusters due to the competing relaxation via the pi-sigma* states.
[J.Chem.Phys. 122, 224320 (2005)]

This work is carried out in the framework of the DFG - Sonderforschungsbereich 450 'Analyse und Steuerung ultraschneller photoinduzierter Reaktionen' Subproject: A4.
Financial support by the Deutsche Forschungsgemeinschaft is gratefully acknowledged.