Disentangling the photophysics of ionized biomolecules in microsolvated environments

Abstract

Water is vital for the existence of life on earth, and therefore, it is dubbed as the matrix of life. Furthermore, the hydration environment plays diverse functional, structural, and dynamic roles in molecular cell biology. For instance, the photophysical and photochemical properties of UV absorbing chromophores found in biological systems are sensitive to their local solvation environment. In hydrated systems, the hydrogen-bond network between the chromophores and surrounding water molecules can provide additional relaxation pathways after excitation or ionization of chromophores, crucial for the photostability of the biological system. On the other hand, the abundant solvent environment around biological matter can enhance radiation-induced damage through, e.g., the radiolysis of water. Despite the huge efforts to unravel realistic dynamics, the role of the hydration environment upon the radiation-induced biological damage remains elusive. In the framework of this thesis, a bottom-up approach is used to systematically investigate the role of a single water molecule in the photophysics of a model chromophore, pyrrole, after ionization. This work can be divided into two main parts: the preparation of pure samples of pyrrole and microsolvated pyrrole-water in the gas phase, and a comparative photophysics investigation on these model systems utilizing time- and position- sensitive detectors.

The preparation of pure and rotationally cold samples of pyrrole and pyrrole-water is achieved using a combination of a cold molecular beam and the electrostatic deflector. A knife-edge for shaping a molecular beam is implemented to improve the spatial separation of the species in a molecular beam by the electrostatic deflector as well as to reduce the contribution of atomic seed gas in the interaction zone. A pyrrole-water cluster beam with unprecedented purity of ~100 % is obtained. The extracted rotational temperature of pyrrole and pyrrole-water after the supersonic expansion is 0.8 ± 0.2 K. Also, the number densities and rotational temperature of the cluster beam is significantly improved.

In the photophysics investigation part, we mimicked radiation damage in pyrrole and pyrrole-water, after outer valence- and inner-shell ionization through strong-field ionization and site-specific x-ray ionization, respectively. In the strong-field case, single- and double- ionization of bare pyrrole resulted in fragmentation of the aromatic ring through breaking the C-C or N-C covalent bonds. Albeit, the protective effects in the cluster, a weak enhanced strong-field ionization resulting in the formation of highly charged ionic species is observed only for the pyrrole-water cluster. In the case of site-specific core-shell ionization, the fragmentation studies on bare pyrrole yielded similar channels as compared to the strong-field ionization scenario. For pyrrole-water, performing a localized x-ray ionization at the aromatic ring resulted in charge- and mass-transfer across the hydrogen-bond. Overall, a hydrogen-bonded single water molecule can facilitate radiation-protection pathways after outer valence- and inner-shell ionization of biomolecules.