Supersonic jet laser induced fluorescence spectroscopy of solvent complexes of tryptophan analogues and molecular mechanics modeling of bare molecule conformers
Description
Solvent complexes of the tryptophan analogues 3-indolepropionic acid, 3-indoleacetic acid, N-acetyltryptophan ethyl ester (NATE), tryptamine, tryptophol, and $\beta$-phenylethylamine, were studied in a supersonic free jet using the technique of laser induced fluorescence. Dispersed emission spectra of some spectral features were obtained. The solvents included water, methanol, ethanol, n-propanol, isopropanol, isobutanol, 2-methyl-2-butanol, 2-methylcyclohexanol, 2,2,2-trifluoroethanol, benzene, acetone, methylene chloride, tetrahydrofuran, acetonitrile, and ammonia. Solvent addition to the polar sites on the 3-indole side chain of the tryptophan analogues produces small, red solvent shifts of the complex origins with small blue shifts occurring in some instances. This is in contrast to the large red shifts observed in indole solvent adducts, which are due to binding at the ring chromophore. Solvent adduct formation strongly favors the polar side chain over the indole ring, especially for the larger solvent molecules. The multiple origin bands of tryptamine, due to different conformers, collapsed to one prominent solvent induced feature by a variety of solvents bearing hydroxyl groups. This behavior was exhibited to a lesser extent by tryptophol. Solvent induced features for the other analogues studied indicated formation of separate complexes for each bare molecule conformer Lifetimes were obtained of the bare molecule conformer peaks of the tryptophan analogues in the origin region. The lifetimes ranged in value from 13.0 to 16.1 ns. Although the range is not great, different conformers did exhibit different lifetimes. Lifetimes were also obtained for the one major solvent induced feature of tryptamine with various hydroxyl bearing solvents. These lifetimes ranged from 14.2 to 15.2 ns The molecular mechanics program MM2(87) was implemented on an IBM mainframe and used to calculate the potential energy surface for the tryptophan and tyrosine analogues tryptamine, 3-indoleacetic acid, 3-indolepropionic acid, tryptophol, $\beta$-phenylethylamine, and tyramine. A series of conformers was found for each analogue, and a Boltzmann distribution was determined for each series. These distributions were matched against the experimentally derived populations to determine the conformer associated with each spectral feature. These results were compared with literature values for tryptamine, 3-indoleacetic acid and 3-indolepropionic acid