The infrared spectra and structures of the valyl-glycine zwitterion isolated in a KBr matrix

The IR absorption spectrum of zwitterions of L-valyl-glycine isolated in a KBr matrix has been measured using a novel dissolution, spray and deposition technique. The number of conformers, their molecular structures and their vibrational spectra have been determined by ab initio molecular orbital calculations of the non-aqueous self-consistent reaction field type, using the Onsager dipole-sphere model at both the density functional theory and Hartree-Fock levels. Six conformers were determined. Although differences in their relative energies were found to be small, one conformer T_g2 was consistently the lowest for a range of cavity radii and for both levels of theory. From comparison of the observed and predicted spectra it is concluded that one or more of the trans group of conformers is present in the KBr matrix. Good agreement between measured and calculated spectra has been obtained for the majority of the prominent bands in the spectrum.