Probing unoccupied electronic states in aqueous solutions

Water is one of the most common compounds on earth and is essential for all biological activities. Water has, however, been a mystery for many years due to the large number of unusual chemical and physical properties, e.g. decreased volume during melting and maximum density at 4 °C. The origin of the anomalies behavior is the nature of the hydrogen bond. This thesis will presented an x-ray absorption spectroscopy (XAS) study to reveal the hydrogen bond structure in liquid water. The x-ray absorption process is faster than a femtosecond and thereby reflects the molecular orbital structure in a frozen geometry locally around the probed water molecules. The results indicate that the electronic structure of liquid water is significantly different from that of the solid and gaseous forms. The molecular arrangement in the first coordination shell of liquid water is actually very similar as the two-hydrogen-bonded configurations at the surface of ice. This discovery suggests that most molecules in liquid water have two-hydrogen-bonded configurations with one donor and one acceptor hydrogen bond compared to the four-hydrogen-bonded tetrahedral structure in ice. This result is controversial since the general picture is that the structure of liquid water is very similar to the structure of ice. The results are, however, consistent with x-ray and neutron diffraction data but reveals serious discrepancies with structures based on current molecular dynamics simulations. The two-hydrogen-bond configuration in liquid water is rigid and heating from 25 °C to 90 °C introduce a minor change in the hydrogen-bonded configurations. Furthermore, XAS studies of water in aqueous solutions show that ion hydration does not affect the hydrogen bond configuration of the bulk. Only water molecules in the close vicinity to the ions show changes in the hydrogen bond formation. XAS data obtained with fluorescence yield are sensitive enough to resolved electronic structure of water molecules in the first hydration sphere and to distinguish between different protonated species. Hence, XAS is a useful tool to provide insight into the local electronic structure of a hydrogen-bonded liquid and it is applied for the first time on water revealing unique information of high importance.

Computational chemistry studies of UV induced processes in human skin

This thesis presents and uses the techniques of computational chemistry to explore two different processes induced in human skin by ultraviolet light. The first is the transformation of urocanic acid into a immunosuppressing agent, and the other is the enzymatic action of the 8-oxoguanine glycosylase enzyme. The photochemistry of urocanic acid is investigated by time-dependent density functional theory. Vertical absorption spectra of the molecule in different forms and environments is assigned and candidate states for the photochemistry at different wavelengths are identified. Molecular dynamics simulations of urocanic acid in gas phase and aqueous solution reveals considerable flexibility under experimental conditions, particularly for for the cis isomer where competition between intra- and inter-molecular interactions increases flexibility. A model to explain the observed gas phase photochemistry of urocanic acid is developed and it is shown that a reinterpretation in terms of a mixture between isomers significantly enhances the agreement between theory and experiment , and resolves several peculiarities in the spectrum. A model for the photochemistry in the aqueous phase of urocanic acid is then developed, in which two excited states governs the efficiency of photoisomerization. The point of entrance into a conical intersection seam is shown to explain the wavelength dependence of photoisomerization quantum yield. Finally some mechanistic aspects of the DNA repair enzyme 8-oxoguanine glycosylase is investigated with density functional theory. It is found that the critical amino acid of the active site can provide catalytic power in several different manners, and that a recent proposal involving a SN1 type of mechanism seems the most efficient one.