The infrared and ultraviolet spectroscopy of tryptophan analogs and their small water clusters: Contributing to the understanding of tryptophan photophysics
The research presented in this thesis characterizes the structural landscapes of various tryptophan analogs and their small water clusters formed in a supersonic expansion via laser spectroscopy and theoretical calculations. The purpose of this research is to contribute to the understanding of the complex photophysics of the amino acid tryptophan through gas-phase molecular spectroscopy. Smaller, more volatile analogs of tryptophan are analyzed to reach this goal, given the complications in vaporizing tryptophan by thermal methods. A powerful combination of spectroscopic methods is used to obtain conformation-specific infrared and ultraviolet spectra of the desired species. Ultraviolet spectra exhibit S1 ← S0 origin transitions of the selected aromatic chromophores, which shift characteristically in energy with conformational changes or upon complexation with solvent molecules. Infrared spectra highlight O–H, N–H, and C–H fundamental vibrations in the hydride stretch region of the infrared (2800–3800 cm−1 ). These stretching vibrational fundamentals are particularly useful in determining structural characteristics of various monomer conformations through changes in their frequencies. They also distinguish cluster structure, as the hydride stretch vibrations are sensitive to the number, type, and strength of hydrogen bond in which they participate since they vibrate directly against the hydrogen bond. Density functional theory calculations contribute to the assignment of infrared spectra to their respective cluster or monomer structures. This thesis summarizes the work completed on indole-(water)n (n = 0–3), 3-methylindole-(water)n (n = 0–1), 1-methylindole-(water) n (n = 0–3), tryptamine-(water)n (n = 0–3), and 3-indolepropionic acid-(water)n (n = 0–3) clusters. Special attention will be given to the water clusters of the indoles, which often lead to a reassessment of the current theory of water's solvation of the chromophores. In addition to water clusters, the studies on tryptamine and 3-indolepropionic acid focus on the conformational flexibility of the two molecules. The implications of these results to tryptophan photophysics will be discussed.
Zwier, Purdue University.
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