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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.

Our experimental set up consists of a fs laser system that provides tunable UV pump and 200nm probe pulses and a magnetic bottle-type PEPICO spectrometer. Two time-delayed fs laser pulses interact with the doubly-skimmed high intensity molecular beam (sample vapor seeded in He or Ar carrier gas) in the ionization region of our 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.

Time-resolved photoelectron spectra of Adenine have been recorded at 246, 251, and 267nm excitation. The two-dimensional data (see Fig. 1) was analyzed by simultaneously fitting the decay dynamics and photoelectron spectra of the contributing relaxation channels. For positive pump-probe delays a double exponential convoluted with a Gaussian provides a good fit for the data. Energy integrated spectra including these fits are shown in Fig. 1 (left attachments) and lifetimes are summarized in Table 1. For all scans, the extracted time constant t₁ is less than 200fs and the lifetime t₂ increases with longer pump wavelengths. Photoelectron spectra associated with the longer time constant can unambiguously be assigned to the S₁ (nπ*) state; extraction of the photoelectron spectra associated with the short time constant is challenging as the fast pump-probe dynamics occur within the time-resolution of our experiment. Spectral features overlap with those of the probe-pump signal and hence their shape strongly depends on the fit parameters used for negative delays. We have started a collaboration with the UGA statistics department for further analysis of our data.

We have also explored the applicability of our set up for studies of larger biomolecular building blocks. The time-of-flight mass spectrum of Adenosine is shown in Fig. 1(d). Two different vaporization techniques have been employed: simple heating inside our pinhole nozzle and a pick-up source. Similar fragmentation pattern were observed with either technique. We are currently investigating other possibilities for vaporization of Adenosine, e.g. a spray jet nozzle.

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 independent research program.

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