Spectroscopy of tryptophan derivatives in supersonic expansions: Addition of solvent molecules

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Abstract
The addition of solvent molecules, primarily water and some alcohols, to isolated tryptophan derivative molecules has been carried out using supersonic free jet techniques. The derivatives studied include 3-indole acetic acid, 3-indole propionic acid, N-acetyltryptophan ethyl ester, tryptamine, and tryptophol. The primary spectroscopic method employed was laser induced fluorescence, with some individual bands also characterized by fluorescence lifetime measurements or dispersed emission spectra. Unlike solvent addition to indoles already studied, these tryptophan derivatives also offer polar addition sites on the 3-indole chain. Addition to these sites, further removed from the electronic transition, produces solvent shifts of the complex origins that are small compared to the red shifts observed in indoles; small blueshifts occur in some instances. Competition between binding at the 3-indole polar group and the indole moiety nitrogen sites is strongly unequal, with the polar 3-indole chain sites favored, especially for larger solvent molecules. In tryptamine, which was studied most extensively, a variety of –OH-bearing solvents were able to collapse the multiple origin bands, due to different conformers, to only one prominent solvent conformer feature. Similar though less pronounced effects were seen in trytophol. Growth of solvent bands was more indiscriminate for the other derivatives studied and was consistent with formation of complexes separately with each bare molecule conformer. The observation of a solvent induced conformation brings up the question of whether tryptophan itself may be ‘‘steered’’ predominantly to one conformation by solution interactions and, if so, whether this plays a role in the molecule’s fluorescence probe properties.