Fundamental studies of flexible biological molecules in the gas phase are of great importance to the biological community. The focus of this work is the ultraviolet and infrared spectroscopy of the neurotransmitter serotonin. The conformational preferences of the monomer, protonated monomer, and water clusters are extensively studied. The conformation-specific ultraviolet and infrared spectra of the monomer and water clusters were compared to that of tryptamine and its water clusters as well as density functional theory to make firm conformational assignments of the eight conformations observed in the supersonic jet for the monomer and four conformations observed in the water clusters. The potential energy surface is also explored by determining energy barrier heights of isomerization from one conformational minimum to another in the monomer via stimulated emission pumping-population transfer and hole-filling spectroscopies. These results were also compared to the energy barriers previously found for tryptamine and the calculated density functional theory energy barriers. Two other biological molecules, tryptamine and melatonin, were studied by dispersed fluorescence where internal mixing of the first and second excited states was found for tryptamine