Cavity Ring-Down Laser Absorption Spectroscopy of Jet-Cooled L-Tryptophan

The jet-cooled ultraviolet direct absorption spectrum of the amino acid tryptophan is reported. The spectrum measured by cavity ring-down laser absorption spectroscopy covers the region where the origin bands of the S-1 <- S-0 transitions of six conformers (A to F) are located. Tryptophan was transferred into the gas phase by two different methods, namely, thermal heating and laser vaporization. The latter technique allowed us to obtain higher densities of tryptophan in the jet at the time when it was probed for spectroscopy. It also avoided thermal decomposition of the sample. On the other hand, a higher signal-to-noise ratio was obtained with thermal heating. Measurements were carried out by laser-induced fluorescence as well. The comparison of the absorption and excitation spectra reveals a higher fluorescence yield and a shorter radiative lifetime for the S-1 state of conformer A relative to the other conformers. Moreover, the comparison of our spectra with each other and with literature data led us to assign a band to a new conformer, which we named G. Finally, the theoretical structure and vibrational frequencies obtained from density functional theory based calculations confirm that the progression observed in the S-1 <- S-0 spectrum of conformer A is consistent with a torsional motion of the amino acid side chain relative to the indole chromophore.