The electronic spectrum of the amino acid tryptophan has been measured in the environment of a cold, supersonic molecular beam. The resonantly enhanced two-photon ionization spectrum of tryptophan shows some features characteristic of more volatile indole derivitives, however the region of the spectrum near the origin shows distinctive low frequency structure absent from the simpler indole containing molecules. The power dependence of the spectrum reveals features which can be attributed to several conformers of tryptophan in the molecular beam. One of these conformers shows a nearly harmonic 26 cm−1 vibrational progression which does not appear in the spectra of other indole derivitives, and the intensity pattern of this progression indicates that this particular conformer undergoes a significant geometry change upon electronic excitation. The lack of many extensive vibrational progressions in the electronic spectrum indicates that the excited state conformers of tryptophan are similar to those in the ground electronic state. The identification of a small number of stable tryptophan conformers is important for understanding the photophysics of tryptophan in solution.

High resolution vibronic spectra of the amino acids tryptophan and tyrosine in 0.38 K cold helium droplets Electronic spectroscopy of tryptophan analogs in supersonic jets: 3Indole acetic acid, 3indole propionic acid, tryptamine, and Nacetyl tryptophan ethyl ester J. Chem. Phys. 84, 6539 (1986); 10.1063/1.450705The electronic spectrum of the amino acid tryptophan in the gas phase

When a picosecond light pulse is incident upon a turbid medium such as tissue, the temporal distribution of diffusely reflected and transmitted photons depends on the optical absorption and scattering properties of the medium. From diffusion theory it is possible to derive analytic expressions for the pulse shape in terms of the optical interaction coefficients of a homogeneous semi-infinite medium. Experimental tests of this simple model in tissue-simulating liquid phantoms of different geometries are presented here. The results of these tests show that, in a semi-infinite phantom, the application of the diffusion model provides estimates of the absorption and transport-scattering coefficients that are accurate to better than 10%. Comparable accuracy was also obtained with this simple model for finite slab, cylindrical, and spherical volumes as long as the objects were of sufficient size. For smaller volumes the absorption coefficient was overestimated because of the significant loss of photons at the bounda ries of the object.

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.

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