Infrared absorption spectra and structures of zwitterions of glycyl-L-alanine and its N-deuterated isotopomer in a KBr matrix

The IR absorption spectra of zwitterions of glycyl-L-alanine and its N-deuterated isotopomer isolated in a KBr matrix have been measured using a novel sampling technique involving dissolution, spray and deposition. In a parallel theoretical study the number of conformers, their IR absorption spectra and structures have been determined by ab initio molecular orbital calculations. The calculations were carried out at both the density functional theory and Hartree-Fock levels, and were of the non-aqueous self-consistent reaction field type, using the Onsager dipole-sphere model. Four conformers were identified. The trans conformer was found to have the lowest energy at all levels of theory and for the range of cavity radii studied. Its structure was found to be closely similar to that of glycyl-L-alanine in the crystal. Comparison of the predicted spectra of the four conformers with the measured spectra showed that both the trans and/or cis conformers could be present in the KBr matrix. Good agreement between measured and calculated spectra, both cis and trans, has been obtained for the majority of the prominent bands in the spectrum.