Abstract
Vickers microindentation hardness of protein crystals was investigated on the (110) habit plane of tetragonal hen egg-white lysozyme crystals containing intracrystalline water at controlled relative humidity. The time evolution of the hardness of the crystals exposed to air with different humidities exhibits three stages such as the incubation, transition, and saturation stages. The hardness in the incubation stage keeps a constant value of 16 MPa, which is independent of the humidity. The incubation hardness can correspond to the intrinsic one in the wet condition. The increase of the hardness in the transition and saturation stages is well fitted with the single exponential curve, and is correlated with the reduction of water content in the crystal by the evaporation. The saturated maximum hardness also strongly depends on the water content equilibrated with the humidity. The slip traces corresponding to the (11 ̅0)[110] slip system around the indentation marks are observed in not only incubation but also saturation stages. It is suggested that the plastic deformation in protein crystals by the indentation can be attributed to dislocation multiplication and motion inducing the slip. The indentation hardness in protein crystals is discussed in light of dislocation mechanism with Peierls stress and intracrystalline water.
Highlights
The knowledge of the mechanical properties of crystals is important for the elucidation of intra-crystalline bonds and practical issues such as the limits of mechanical stability [1,2].The mechanical properties of protein crystals is greatly affected by water content, dislocations still play a crucial role in plastic deformation
We have shown the indentation hardness of T-hen egg-white lysozyme (HEWL) crystals with intracrystalline water under controlled relative humidities
The hardness strongly depends on the water content in the crystals associated with the evaporation and humidity
Summary
The knowledge of the mechanical properties of crystals is important for the elucidation of intra-crystalline bonds and practical issues such as the limits of mechanical stability [1,2].The mechanical properties of protein crystals is greatly affected by water content, dislocations still play a crucial role in plastic deformation. Our understanding of the mechanical properties of protein crystals is poor compared with those for metal and covalent crystals. There are interesting studies on the mechanical response to the hydration of biological materials such as bone by using micro- and nano-indentation techniques [3,4,5,6]. Such mechanical properties in hydrated biomaterials seem to be partially similar to those in protein crystals, they are non-crystals
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