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

Resonant ion-dip infrared (RIDIR) and UV−UV hole-burning spectroscopies are used to record the hydride stretch infrared spectra and S 1 ←S 0 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 S 1 ←S 0 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 (anti, gauche toward the phenyl side, and gauche toward the pyrrole side of indole) and three amino group orientations (180°, ±60°) at each position. All three anti conformers are observed experimentally, whereas only the two lowest-energy of the three orientational conformers at each gauche position are observed. The dominant factor in determining the form of the CH stretch infrared spectrum is the orientation of the amino group, with the amino group position playing a secondary role. The frequencies of the S 1 ←S 0 origin transitions, on the other hand, are most sensitive to the position of the amino group, whether it is anti, gauche phenyl, or gauche pyrrole. The prospects for using these methods more generally to characterize the conformations and energetics of flexible biomolecules are discussed.