Electronic spectroscopy of tryptophan analogs in supersonic jets: 3-Indole acetic acid, 3-indole propionic acid, tryptamine, and N-acetyl tryptophan ethyl ester

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Abstract
The electronic spectroscopy of four different tryptophan analogs, 3‐indole acetic acid, 3‐indole propionic acid, tryptamine, and N‐acetyltryptophan ethyl ester (NATE) has been studied in a supersonic molecular beam using laser‐induced fluorescence and resonantly enhanced two‐photon ionization. The electronic transition to the lowest excited singlet state occurs at 35 039, 34 965, 34 918, and 34 881±2 cm−1 for 3‐indole acetic acid, 3‐propionic acid, tryptamine, and NATE, respectively. The relatively small differences in the electronic origin transition frequencies suggests that the lowest excited singlet state for all of these moelcules is the 1Lb state. The spectra reveal that each of these molecules have stable conformers in the gas phase, analogous to our previously reported studies of tryptophan. A low frequency vibrational mode has been observed in 3‐indole propionic acid, tryptamine, NATE, and tryptophan which involves motion of the side chain against the indole ring. We have observed that forming a van der Waals complex between tryptamine and a single methanol molecule causes the spectral features due to different conformers of the free molecule to collapse to a single line, suggesting that one particular conformer becomes the most stable species. This emphasizes the importance of including solvent interactions in any attempt to model the behavior of these molecules in solution.