Many biological processes are controlled by the interactions between proteins and their environment. Fluorescent probes provide one of the best tools for the study of protein dynamics and structural conformations. In spite of their wide application, there is in general little understanding of the effects of environment on the optical properties of the chromophores themselves. It is well established that the fluorescence and phosphorescence spectra of proteins are due only to three amino acids - tryophan, tyrosine, and phenylalanine. The basic aromatic ring systems in tryptophan and tyrosine are indole and phenol, respectively. We propose to study the effects of microscopic solubility and hydrogen bonding on the photophysics of substituted phenols and indoles. We shall use laser induced multiphoton ionization time of flight mass spectroscopy (MPI-MS) as well as photoelectron spectroscopy (MPI-PES) to probe the excited state energetics and dynamics of these fluorescent chromophores of proteins. By studying clusters of substituted phenols and indoles with different solvent molecules in supersonic beams, we shall mimic the emission from proteins in the absence of intermolecular interactions. MPI-MS and MPI-PE spectroscopy in supersonic beams provide us with a powerful method of isolating the relative contributions of molecular interactions such as hydrogen bonding, exciplex formation, excited level inversion, etc. By controlling the relative concentrations of the solute and solvent in the supersonic beam, and by monitoring specific masses in the time of flight mass spectrometer, we shall systematically study the radiative and nonradiative processes that dominate at different stages of solvation. Furthermore, we propose to study the lifetimes of the excited states of monomers and clusters, as well as the vibrational state distributions of their ions as a function of excitation energy. Such a study will provide us with a measure of the binding energies of the """"""""isolated"""""""" clusters in the ground and first excited singlet and triplet states. This will be completely new information about the strength of such interactions which play a central role in determining the properties of proteins.
