Laser Spectroscopy of Jet-Cooled Biomolecules and Their Water-Containing Clusters:  Water Bridges and Molecular Conformation

The techniques of laser-induced fluorescence (LIF), resonant two-photon ionization spectroscopy (R2PI), UV−UV hole-burning spectroscopy, fluorescence-dip infrared spectroscopy (FDIRS), and resonant ion-dip infrared spectroscopy (RIDIRS) have been used to study the ultraviolet and infrared spectra of individual conformations of small, flexible biomolecules cooled in a supersonic expansion. The water-containing clusters of these molecules and of other rigid molecules that possess multiple H-bonding sites are considered. The water molecules in many of the solute−(water)_n clusters form hydrogen-bonded bridges between donor and acceptor sites on the solute molecule. The infrared spectroscopy of these bridges has been explored in some detail. Water bridges are also formed when one of the H-bonding sites is on a flexible side chain. These bridges have a profound influence on the conformational preferences of the flexible biomolecules.