Conformational and infrared spectral studies of L-methionine and its N-deuterated isotopomer as isolated zwitterions

The mid-IR spectra of L-methionine and its N-D-3 deuterated isotopomer as isolated zwitterions in a KBr solid solvent have been obtained by means of a newly developed infrared sampling technique, which involves the production of thin films. Ab initio calculations of the nonaqueous self-consistent reaction field type, using the Onsager dipole-sphere model have been carried out at both the Hartree-Fock and density functional levels of theory for the prediction of the vibrational spectra and molecular structures of Met zwitterions isolated in KBr matrixes. Seven stable conformers of Met zwitterion were located, of which three were shown to be characterized by strong intramolecular H-bonding. Another conformer was found to have the same conformation, and similar molecular structure, as one of the two forms of Met crystals. The calculated structural data supplement the limited crystal data obtained by X-ray diffraction studies. Comparison of the calculated spectra of the seven conformers with the measured spectra favors the presence of zwitterions of conformer GtG in the KBr solid solvent. However, the evidence against a number of the other conformers, in particular, gtt and tGG, is not overwhelming.