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

Abstract

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.

Rotational Spectrum of Tryptophan

“…Hence, a final comparison between the experimental and theoretical values of the quadrupole coupling constants (Tables III and IV, respectively) serves to discriminate between these conformers and allows the unequivocal identification of the observed rotamer as conformer IIc+1. This conformer has never been reported in any previous studies of Trp, 3,[22][23][24][25] although IIc+2 conformer -structurally very similar to IIc+1 -has been proposed. 23,28 It seems that the spectral signatures of conformer IIc+1 has been misleadingly attributed to the existence of the conformer IIc+2, based on the relative energies 23 and conformational interconversion processes.…”

“…Recently, a cavity ring-down absorption experiment observed seven tryptophan conformers, adding a new one to the six previously reported. 25 No reports on the rotational spectrum of tryptophan had been published up to now.…”

Laboratory Studies on the Role of PAHs as DIB Carriers

“…When the PAHs increase in size, the temperature needed to achieve a proper vapor pressure also increases. As the high temperature may cause the decomposition of the molecules, laser vaporization can serve as an alternative (Rouillé et al 2009a).…”

Structural Dynamics of Free Amino Acids in Diffraction

“…Unlike the indole chromophore, l-tryptophan has a "floppy" peptide moiety, which contains three torsional degrees of freedom ( Figure 1); they allow for the formation of multiple conformers. The spectroscopic [5][6][7][8][9][10][11][12] as well as the quantum chemical calculations [13,14] of these conformers have been the subject of many studies. Indeed, multiple stable conformers have been identified in a jet-cooled molecular beam by the Levy group, [6][7][8] and in a later work, the fluorescence lifetime of photoexcited l-tryptophan was shown to be conformer-dependent.…”