Assignment problems of amino acids,di- and tripeptides and proteins in the near infrared region

Nine amino acids, four dipeptides, one tripeptide and several types of proteins were studied by reflectance and absorption FT-IR spectroscopy. Mid-IR and, where necessary, Raman frequencies were applied to assign the combination and overtone bands occurring in the near-IR region. On one occasion, an isotope effect (deuterization) was also employed for the recognition of the combinations involving N⁺H₃ vibrations. The overtone and combination bands of N⁺H₃, CH₃, CH₂, NH₂ and amide groups were, in part, assigned. On the basis of this assignment and with the use of second derivative and/or Fourier deconvolved spectra, attempts were made to recognize the characteristic features of glutamine and asparagine components of some proteins. The IR spectra were taken on an FT-IR spectrometer with a configuration allowing spectra to be measured in the 10,000 to 400 cm⁻¹ range.