The Infrared and Ultraviolet Spectra of Individual Conformational Isomers of Biomolecules: Tryptamine

Abstract

Resonant ion-dip infrared (RIDIR) and UV−UV hole-burning spectroscopies are used to record the hydride stretch infrared spectra and S1←S0 ultraviolet spectra of each of seven conformational isomers of tryptamine free from interference from one another. The different conformations of the ethylamine side chain produce unique S1←S0 vibronic spectra, which can serve as the basis for RIDIR spectroscopy. The seven conformers possess unique spectral signatures in the alkyl CH stretch region of the infrared, which aid in the assignment of the observed transitions in the ultraviolet. Density functional theory (DFT) calculations of the structures, relative energies, and harmonic vibrational frequencies of nine low-energy minima are compared with the present and previous experimental data on tryptamine to assign the spectra of all seven conformers, all of which point the α carbon out of the plane of the indole ring. The nine conformers consist of all combinations of the three minimum-energy amino group positions (an...