Understanding the deformation of materials at the nanoscale level provides insights into the incipient deformation process. Additionally, comprehending how a material deforms directly improves understanding of its mechanical properties. For atomic crystals such as metals and ceramics, direct observation of nanoscale deformation has clarified how materials deform and fracture while revealing the key factors governing the deformation process. However, in systems where the constituent elements themselves are significantly larger, understanding the deformation process is important not only for theoretical framework development but also for gaining new insights into deformation mechanisms. In this study, we investigated the deformation mechanisms of macromolecular crystals using protein crystals as a model, which consist of significantly larger molecular components than conventional atomic or small-molecule crystals. Through microscale indentation observation via x-ray topography, we revealed that plastic deformation occurs via the nucleation and motion of dislocations, which can be explained by the conventional dislocation theory. Nanoindentation experiments at a scale smaller than the molecular size of macromolecular crystals, along with estimations of the activation volume of dislocations, demonstrated that the molecules deform slightly at the incipient stage before dislocation nucleation. This finding indicates that the size of the constituents in materials affects material deformation, and it suggests the potential for expanding deformation mechanisms by considering the characteristics of the constituent components.

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• Crystal defects
• Disclinations & dislocations
• Mechanical deformation
• Plastic deformation