Fingerprint IR Spectroscopy to Probe Amino Acid Conformations in the Gas Phase

Abstract

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...

Importance of Entropy in the Conformational Equilibrium of Phenylalanine: A Matrix-Isolation Infrared Spectroscopy and Density Functional Theory Study

“…27,28 Most of these studies also reported theoretical calculations at various levels of theory. [5][6][7][8][9][10][11][12][13]15,17,[19][20][21][22][23][24][25][26][27][28] In general, the gas-phase results are qualitatively consistent with the matrix-isolation data, despite van der Waals forces appearing in the solid matrix between the sample and inner gas molecules. The most stable structures of aliphatic amino acids have been found to be stabilized by a bifurcated H-bond linking the carboxylic oxygen atom and the hydrogen atoms of the amino group.…”

“…In proteins, amino acids occur as residues of neutral L-R forms, whereas the zwitterionic structures are found in the crystalline state and in solution. [1][2][3] Amino acids adopt their neutral form also in the gas phase [4][5][6][7][8][9][10][11][12][13] and in low-temperature inert matrixes. [14][15][16][17][18][19][20][21][22][23][24][25][26][27][28] Therefore, the structures of these compounds have been extensively studied in both these media.…”

“…However, existence of other conformers has been proven experimentally for all molecules studied hitherto. [4][5][6][7][8][9][10][11][12][13][15][16][17][18][21][22][23][24][25][26][27][28] The conformational distribution of amino acids depends strongly on the experimental conditions, because they have low conformational interconversion barriers. Low barriers have been predicted theoretically for glycine and alanine (for alanine, as low as 129 cm -1 and less than 35 cm -1 for the internal rotations around C-N and C-C(dO) bonds, respectively).…”

“…For Trp, three conformers could be characterized by IR ion-dip spectroscopy and DFT calculations. 31 Phe and Trp were also studied previously in a supersonic beam by UV and IR ion-dip spectroscopy combined with ab initio computations, by Snoek et al 9,12 These studies centered on the analysis of the O-H and N-H stretching vibrations, and an extensive computational analysis was presented. Six conformers of each compound were found to contribute to the corresponding UV-UV hole-burning spectra, whereas five and six conformers were recognized in the ion-dip IR spectra of Phe and Trp, respectively.…”

“…More recently, ultraviolet rotational contour analysis studies brought a new assignment of Phe conformers. 11 Despite the important discrepancies found in the different studies previously reported, 9,11,12,31 there is nowadays a general consensus relative to the fact that the most stable conformer in both Phe and Trp is stabilized by a relatively strong intramolecular H-bond established between the carboxylic OH group and the nitrogen atom. The increased stability of this conformer, compared to what is found for aliphatic amino acids, was explained by the existence of a "daisy-chain" sequence of interactions involving aromatic π-electrons, the amino group and the carboxylic OH group.…”