Infrared Spectra and Structures of the Valyl-Alanine and Alanyl-Valine Zwitterions Isolated in a KBr Matrix

Abstract

The infrared absorption spectra of the dipeptide zwitterions l-valyl-l-alanine and l-alanyl-l-valine have been measured using a novel infrared sampling technique. The two dipeptides are isomers, and similar spectra are expected and were measured. With the better resolved spectra obtained with the dissolution, spray, and deposition technique, small but significant differences were identified, not evident in the normal KBr pellet spectra. Ab initio molecular orbital calculations of the self-consistent reaction field type using the Onsager dipole−sphere model were undertaken for structure determinations and spectral predictions at Hartree−Fock and density functional theory levels. A number of conformers were identified; the structure of the calculated most stable one of l-valyl-l-alanine was found to be in excellent agreement with the X-ray crystallographically determined bond length and angles. The spectral resolution obtained, together with theoretical predictions of the absorption spectra, and comparisons with the spectra of related dipeptides and amino acids have enabled vibrational assignment of bands and identification of molecular conformers.