Viscoelastic properties of protein crystals: Triclinic crystals of hen egg white lysozyme in different conditions

A technique for the measurement of the dynamic Young's modulus E and logarithmic decrement 𝓋 of protein crystals and other microscopic samples by the resonance method modified to a microscale is described. Monoclinic crystals of horse hemoglobin and sperm whale myoglobin; triclinic hen egg white lysozyme crystals, crosslinked by glutaraldehyde; and native and crosslinked needlelike lysozyme crystals were studied, as were amorphous protein films. The E of wet protein crystals is shown to be in the range (3–15) × 103 kg/cm2, 𝓋 = 0.3–0.7. The crosslinking does not significantly affect the values. General elastic properties were analyzed for triclinic lysozyme crystals. No frequency dependence of E and 𝓋 was found over the frequency range of 2.5–65 kHz. The temperature dependence was found to be characteristic for glassy polymers; the decrement of Young's modulus was −2.4 ± 0.1%/°C. The guanidine HCl denaturation caused a 1000‐fold decrease of E, its temperature dependence becoming similar to that of rubberlike materials. Studies of pH and salt effects showed E to be influenced by ionization of the acidic groups at pH 3–4.5. A humidity decrease from 100 to 30% caused a three‐ to fourfold increase of E and a decrease of 𝓋. The final values of E = (40–60) × 103 kg/cm2 and 𝓋 ≃ 0.1 were the same for dry crystals and amorphous films, whether crosslinked or not. These values may be attributed to the protein globular material.