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We report the infrared (IR) absorption spectra of different conformational isomers of gas phase amino acid molecules in the molecular fingerprint region of 330-1500 cm ÿ1 . The IR absorption spectra for three conformers of the amino acid tryptophan show absorption bands that uniquely identify the conformational structure of the molecule and that are well matched by density functional theory calculations. The present observations hold great promise for future identification of conformational folding of larger molecules by means of their IR absorption characteristics. DOI: 10.1103/PhysRevLett.91.203003 PACS numbers: 33.20.Ea, 31.15.Ew, 33.15.Bh, 33.20.Tp Over the past 20 years, sophisticated techniques have been developed to bring intact large biomolecules into the gas phase. The techniques most widely used today are matrix-assisted laser desorption ionization (MALDI) [1] and electrospray ionization (ESI) [2]. Because of their ability to provide mass and sequence information of biological samples, both techniques have revolutionized analytical biochemistry. However, inferring structural information on gas phase ions and molecules is often a difficult task. The knowledge of the gas phase structure can provide important insight into fundamental intramolecular interactions and can serve as a calibration point for theoretical models. While elaborate tools for solid and liquid samples exist, most of them cannot be applied to gas phase samples. One of the few techniques that can provide direct structural information on gas phase species is ion mobility, in which large ionized molecules are mass selected and pass through a drift cell filled with a buffer gas [3,4]. The molecular ions will arrive at the end of the drift cell after a period of time which depends on their collisional cross section. Information can also be obtained using spectroscopic methods, either in the ultraviolet (UV) [5] or in the infrared (IR) [6,7]. The IR absorption spectrum is a unique identifier for the structure: line intensities and frequencies give direct information on the forces that hold the molecule together. Recently, it was experimentally demonstrated that additional information can be obtained by the direct measurement of the direction of the vibrational transition dipole moment of isolated species [8]. Selective IR induced isomerization in the gas phase is another interesting technique that provides structural and dynamical information [9]. We here present IR absorption spectra for three conformers of tryptophan in the gas phase over a wide frequency range, from 330 to 1500 cm ÿ1 . It is observed that the conformation has a strong influence on the spectra. The experimental results are combined with density functional theory (DFT) calculations, and unique information on the structures of the individual conformers is obtained.IR absorption spectra of gas phase species can be obtained by ion-or fluorescence-dip spectroscopy [6,7,10], a technique that has been applied to various interesting systems, such as isolated [11] or paired nucl...

The IR absorption spectrum of the jet-cooled benzoic acid monomer and dimer have been recorded throughout the 500-1900 cm Ϫ1 range via ion dip spectroscopy. Both spectra show a wealth of vibrational modes and the monomer spectrum is remarkably different from that of the dimer. Density functional theory calculations show quantitative agreement with the experimental data. The C-O-H out-of-plane bending vibration in the dimer is poorly reproduced in the theoretical calculations and a more accurate description of the doubly hydrogen bonded structure is therefore still needed.