Effect of intracrystalline water on micro-Vickers hardness in tetragonal hen egg-white lysozyme single crystals

Abstract

Mechanical properties of high quality tetragonal hen egg-white lysozyme single crystals which are one type of protein crystal were investigated by the indentation method. The indentation marks were clearly observed on the crystal surface and no elastic recovery of them occurred. The value of the micro-Vickers hardness in the wet condition was estimated to be about 20 MPa at room temperature. The hardness greatly depended on the amount of intracrystalline water (mobile water) contained in the crystals. The hardness increased with increasing evaporation time to air at room temperature. It reached the maximum at about 260 MPa, which is 13 times as much as that in the wet condition. The origin of such a change in hardness was explained in terms of the dislocation mechanisms in lysozyme single crystals.

Microindentation Hardness of Protein Crystals under Controlled Relative Humidity

“…Note that the exposure of the crystal to air can lead to the evaporation of the intracrystalline water. The behavior of hardness exhibits three stages with increasing exposure time, as reported previously [29]. First stage is the incubation stage in which the magnitude of hardness keeps a constant value even during the water evaporation, where the indented plane is still kept in wet condition.…”

“…The value of the hardness in the incubation stage is found to be 16 MPa, as seen in Figure 1. This value is slightly lower than that reported previously [27][28][29]. The reason can be attributed to the high accuracy for the measurements at controlled humidity.…”

“…From data points in Figure 1, it is noted that the scatter of measured values in each stage, especially transition and saturation stages, is less than that reported elsewhere [27,29]. The low scattering of measured values is attributed to controlled relative humidity in this work.…”

“…The studies on plastic properties of protein crystals have been carried out by using Vickers microindentation method, mainly with T-HEWL ones [27][28][29]. In the wet condition, the indentation marks were clearly observed on the (110) crystal plane.…”

“…The average activation energy of the dislocation motion has been also evaluated to be 0.6 eV from the measurements of the temperature dependence of the indentation hardness [28]. Furthermore, it has been found that the indentation hardness increases with the evaporation of the intracrystalline water in open air where the evaporation time dependence of the hardness has three stages such as incubation, transition, and saturation stages [29]. The maximum value of the hardness in the dried condition has been about one order of magnitude larger than that in the wet condition.…”