Mechanical properties such as elastic and plastic characteristics of protein crystals are important for the understanding of the intermolecular interaction and growth mechanism. However, there are very few studies on the mechanical properties of protein crystals because of the difficulty of the crystallization and handling. In this review, we discuss the measurements of elastic constants in protein crystals by ultrasonic pulse-echo method. All elastic constants in cross-linked hen egg-white lysozyme crystals with tetragonal structure at 98% relative humidity are determined to be C_<11>=C_<22>=5.50GPa, C_<12>=4.33GPa, C_<13>=C_<23>=3.94GPa, C_<33>=5.22GPa, C_<44>=C_<55>=0.68GPa, and C_<66>=0.84GPa, respectively. The elastic constants, especially shear components, of protein crystals are much smaller than those of organic molecular crystals with van der Waals bonding. The small elastic constants of protein crystals can be attributed to the intracrystalline water, especially mobile water, in the crystals. The control of the mobile water in protein crystals can lead to the growth of high quality crystals and the improvement of the crystal properties.
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