FTIR spectra of solid poly-l-lysine in the stretching NH mode range

Three bands at 3270 cm− 1, 3200 cm− 1 and 3030 cm− 1 are found in the IR stretching proton (ν1) mode spectral range in spectra of solid poly-l-lysine (PLL). Strong quantitative changes of these bands are observed in samples dried from water solutions with different pH. The narrow band at 3270 cm− 1, which is strong in the spectrum of PLL precipitated from pH = 12 alkaline medium, is assigned to the ν1 peptide proton mode of NH-CO (amide A) of the β-sheet structure type. The band at 3200 cm− 1, which is intensified in PLL precipitated from pH = 1 acidic medium, relates to the ν1 peptide mode in the random coil structure. The band at 3030 cm− 1, whose peak intensity increases two-fold in going from alkaline to acidic medium, is assigned to the ν1 modes of protonated NH3+ side chain groups. The frequencies of all bands were used for estimating H-bond energy relying on an empirical correlation between this property and the red shift of the ν1 band. The enthalpy of the secondary structure transition from β-sheet to the random coil, which is observed in PLL at the change of pH from 11 to 1 amounts to 4.7 kJ mol− 1.