Conformational Behavior of α-Alanine. Matrix-Isolation Infrared and Theoretical DFT and ab Initio Study

Abstract

Two conformers of the nonionized α-alanine and its isotopomer N,N,O-d 3-alanine have been observed in low-temperature Ar matrixes. Their infrared spectra have been analyzed and assigned using DFT/B3LYP/aug-cc-pVDZ and MP2/aug-cc-pVDZ geometry and frequency theoretical calculations. Two different intramolecular H-bonds, bifurcated NH2···OC and N···H−O, were found in the observed α-alanine conformers, I and IIa. We found that the DFT/B3LYP/aug-cc-pVDZ method yields vibrational frequencies of the α-alanine conformers in excellent agreement with the experimental data.

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 the previous studies of matrix-isolated glycine, alanine and valine, 17,22,24 it was found that these amino acids tend to decompose at strong heating. Thus, sublimation temperature had to be kept as low as possible to avoid decomposition of the compounds, but high enough to allow their sublimation.…”

“…[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. A number of aliphatic amino acids as well as some of their derivatives have been investigated experimentally up to date: glycine, 4,5,[14][15][16][17][18] sarcosine (N-methylglycine), 19 N,N-dimethylglycine, 20 alanine, 6,[21][22][23] valine, 7,24 proline 8,25,26 and serine.…”

“…[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. A number of aliphatic amino acids as well as some of their derivatives have been investigated experimentally up to date: glycine, 4,5,[14][15][16][17][18] sarcosine (N-methylglycine), 19 N,N-dimethylglycine, 20 alanine, 6,[21][22][23] valine, 7,24 proline 8,25,26 and serine. 27,28 Most of these studies also reported theoretical calculations at various levels of theory.…”

“…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).…”