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 (nu(1)) 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 nu(1) peptide proton mode of NH-CO (amide A) of the beta-sheet structure type. The band at 3200 cm(-1), which is intensified in PLL precipitated from pH=1 acidic medium, relates to the nu(1) 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 nu(1) modes of protonated NH(3)(+) 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 nu(1) band. The enthalpy of the secondary structure transition from beta-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).