Back to Table of Contents

44110-G6
On the Photostability of Biomolecular Building Blocks and Their Clusters: The Role of pi sigma* States in Non-Radiative Decay

Susanne Ullrich, The University of Georgia

Exposure to ultraviolet (UV) radiation present in the sun's spectrum or in artificial light sources is a major health risk due to adverse effects such as sunburn, skin aging, and skin cancer. Among a number of other chromophores found in human skin, the deoxyribonucleic acid (DNA) bases are strong absorbers of UV radiation. As the building blocks of nucleic acids the photophysical and photochemical properties of the DNA bases are implicated in the photostability of our genetic coding material. To be effective, photoprotective deactivation mechanisms must operate on ultrafast time scales in order to dominate over competing photochemical processes that potentially lead to destruction of the biomolecule. Using molecular beam techniques combined with time-resolved spectroscopy, the excited state relaxation dynamics in isolated DNA bases can be investigated in unprecedented detail free from interactions with the surroundings or within a well-defined micro-environment (e.g. water clusters or base pairing). Femtosecond time-resolved photoelectron spectroscopy (TRPES) provides unique capabilities for studying photoinduced processes in polyatomic molecules. Partial ionization probabilities for ionization into cationic states of specific electronic character can differ drastically with respect to the molecular orbital nature of the neutral excited state. Hence a PES obtained via a two step excitation-ionization scheme provides a distinct fingerprint of the neutral excited state. Changes in the PES, observed as the delay between the pump and probe pulses is scanned, can be associated with electronic configurational changes during the relaxation process. This spectroscopic information only obtainable through TRPES has proven crucial in discerning complex relaxation dynamics involving competing deactivation pathways. Combined with time-of-flight mass spectrometry using coincidence detection (PEPICO – photoelectron photoion coincidence) a TRPES spectrum of a certain molecule/cluster size can be recorded even if a sample mixture is present in the molecular beam. We have recently completed the installation of our fs laser system that provides tunable UV pump and 200nm probe pulses and we have constructed a magnetic bottle-type PEPICO spectrometer. The time-delayed fs laser pulses interact with the doubly-skimmed high intensity molecular beam (sample seeded in He or Ar carrier gas) in the ionization region of the PEPICO spectrometer. The photoelectron spectrometer uses a strong, highly divergent magnetic field produced by an axially magnetized permanent ring magnet that meets a weak guiding field to form a magnetic ‘bottle' allowing for a high collection efficiency of photoelectrons. Ion detection is based on a modified Wiley-McLaren linear TOF mass spectrometer that accommodates the ring magnet for the photoelectron spectrometer. Ions are extracted from the ionization region with a positive high voltage towards ground, pass through the center hole of the ring magnet, and are then accelerated towards a negative high voltage; the 80 cm long field-free flight tube is kept at the same negative potential. Figure 1 shows spectra that were recorded with our new set up for characterization and optimization of the spectrometer performance. Currently, we are using this set up to record TRPES spectra of the DNA bases at different excitation wavelengths to determine onsets and branching ratios of competing deactivation channels. A TRPES spectrum of adenine recorded at 250nm excitation and two-photon 400nm probe is also displayed in Fig.1. Graduate and undergraduate students have been actively engaged in setting up our new laser laboratory and gained expertise in state-of-the-art techniques and instrumentation of modern laser spectroscopy. S.U. and her research group have greatly benefited from the ACS PRF G Starter grant during these initial stages of establishing our research program.

Back to top