The low frequency vibrations of the alkylamino backbone of microhydrated tryptamine have been studied using dispersed fluorescence spectroscopy and ab initio calculations and compared to the corresponding vibrations of bare tryptamine. The complete spectrum of torsional and librational motions of the backbone and a number of intermolecular tryptamine-water vibrations could be determined and assigned. The change of vibrational frequencies due to force constant change of the backbone vibrational motions upon conformational locking via microhydration and change of the reduced masses by the water attachment will be discussed. (c) 2009 Elsevier B.V. All rights reserved.