Probing the structure and dynamics of jet-cooled biomolecules: Laser spectroscopy of water bridges, hydrogen -bonded dimers, and conformationally flexible molecules

The research presented in this thesis focuses on understanding the nature of the complex potential energy landscapes that characterize small biomolecules and their molecular clusters using a powerful combination of supersonic expansion cooling and laser spectroscopic techniques. The work presented here probes three main aspects of the chemical physics of biomolecules and their analogs—the structure and dynamics of the bare molecule, solute-solute interactions, and solute-solvent interactions. The first part of this thesis focuses on the keto-enol tautomers, 2-pyridone and 2-hydroxypyridine, and their small water clusters. Water molecules form bridges that span the H-bond donor and acceptor sites of the rigid tautomers, resulting in infrared spectra that reflect the strong coupling along the bridge, both in the form of the hydride stretch normal modes and in the breadth of the observed transitions. The second part of this work focuses on understanding the consequences of forming multiple, strongly H-bonds in a prototypical dimer species—benzoic acid dimer and its isotopomers. The infrared spectra of these dimers are extremely complex, reflecting the highly anharmonic nature of the OH stretching vibration. Results of model calculations of the anharmonic coupling responsible for the complex infrared spectrum are presented. Finally, results on the conformational landscape, isomerization dynamics, and sequential solvation of the neural hormone melatonin are presented. The ultraviolet and infrared spectroscopy of melatonin identifies the presence of five distinct conformations of melatonin, whose structures have been determined. The changes in the potential energy surface of melatonin upon sequential solvation have been addressed in studies of the spectroscopy of melatonin-(water)1,2 clusters. The dynamics of conformational isomerization following single-conformation vibrational mode specific excitation of melatonin and melatonin-(water) 1 clusters have also been investigated