Mechanical properties such as plasticity are fundamental and important properties in processing and applications of materials. Protein crystals are one of the macroscopic molecular crystals composed of protein molecules with nanometer size. The application of protein crystals has been explored not only for the structure analysis of proteins, but also for their application as solids. Generally, it is known that native protein crystals are quite brittle. However, it is empirically known that the mechanical property of protein crystals is enhanced by a cross-linking technique. One of the qualitative reasons is that chemical cross-linking enhances the mechanical strength, but the detailed mechanical properties are not clear. Herein, we investigate the macroscopic elastic and plastic behaviors of cross-linked tetragonal hen-egg-white lysozyme crystals under a compression test. The cross-linked crystals exhibit plastic deformation whereas the native crystals show brittle fracture. The plastic behavior of the cross-linked crystals clearly shows yield phenomena with upper and lower yield points. Stress-induced dislocations are also noted. The Burgers vectors of the moving dislocations of the crystals are characterized by synchrotron x-ray topography, and the results indicate that the slip systems are controlled by cross-linking. Thus, cross-linking leads to the macroscopic plastic deformation of protein crystals. The ductile properties of the cross-linked protein crystals indicate their great potential for material applications such as biocatalysis and biosensing.

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• Crystal defects
• Elastic deformation
• Elasticity
• Mechanical deformation
• Plastic deformation
• Plasticity
• Biological materials
• Cross-linked polymers
• Crystalline systems
• Macromolecules
• Crystallization
• Mechanical testing
• X-ray diffraction
• X-ray imaging