A correlation between the proton stretching vibration red shift and the hydrogen bond length in polycrystalline amino acids and peptides

The FTIR spectra of pure and isotopically diluted (H/D and D/H) polycrystalline L-glutamine, L-hystidine, L-tyrosine, DL-serine, L-threonine, di-, tri-glycine and di-glycine ·HC1 ·H₂O salt were measured in the range 4000-2000 cm^-1 at temperatures from 300 to 10 K. The frequencies of decoupled proton stretching mode bands ν₁, which can be observed only at low temperature, were used for estimation of the of ν₁-bands red shift, which occurs upon formation of H-bonds involving ionized NH₃⁺ and/or peptide HN-CO groups. The empirical correlation between the red shift and H-bond length, which was found previously for binary gas phase H-bonded complexes, carbohydrates and nucleosides [M. Rozenberg, A. Loewenschuss and Y. Marcus, Phys. Chem. Chem. Phys., 2000, 2, 2699-2702; M. Rozenberg, C. Jung and G. Shoham, Phys. Chem. Chem. Phys., 2003, 5, 1533-1535], was now extended to H-bonded networks in polycrystalline amino acids and peptides. The energies of the different H-bonds present in the crystalline structures could also be successfully estimated from the well-established empirical correlation [A. V. logansen, Spectrochim. Acta, 1999, A55, 1585-1612] between this property and the red shifts of the corresponding ν₁ mode bands.