Temperature-dependent dielectric properties of slightly hydrated horn keratin

With an aim to reveal the mechanism of protein–water interaction in a predominantly two phase model protein system this study investigates the frequency and temperature dependence of dielectric constant ɛ′ and loss factor ɛ″ in cow horn keratin in the frequency range 30 Hz to 3 MHz and temperature range 30–200 °C at two levels of hydration. These two levels of hydration were achieved by exposing the sample to air at 50% relative humidity (RH) at ambient temperature and by evacuating the sample for 72 h at 105 °C. A low frequency dispersion (LFD) and an intermediate frequency α-dispersion were the two main dielectric responses observed in the air-dried sample. The LFD and the high frequency arm of the α-dispersion followed the same fractional power law of frequency. Within the framework of percolation cluster model these dispersions, respectively have been attributed to percolation of protons between and within the clusters of hydrogen-bonded water molecules bound to polar or ionizable protein components. The α-dispersion peak, which results from intra-cluster charge percolation conformed to Cole–Cole modified Debye equation. Temperature dependence of the dielectric constant in the air-dried sample exhibited peaks at 120 and 155 °C which have been identified as temperatures of onset of release of water bound to polar protein components in the amorphous and crystalline regions, respectively. An overall rise in the permittivity was observed above 175 °C, which has been identified as the onset of chain melting in the crystalline region of the protein.