The stress-induced reordering of a protein crystal, distorted by electron irradiation, was observed by scientists at the Fritz Haber Institute of the Max Planck Society in Berlin/Germany at very low temperatures, where thermal activation is absent (Nature 6 May 1999).
Stress-induced ordering offers an additional pathway for the formation of order in an system of particles at low temperatures. Usually the ordering requires substantial mass transport of the constituents, which is frequently suppressed by a substantial activation barrier. Thermal activation can surmount this barrier. But, increasing the temperature very often also changes the thermodynamically stable phase, i.e., the structure itself.
In the present study the authors observed the reordering of a crystal without thermal activation. A crystal of the connector protein of a bacteriophage was irradiated by electrons during the observation in a transmission electron microscope at low temperature. The stress, resulting from the radiation damage, was eventually high enough to drive mass transport. The order quickly reemerged, before after longer irradiaton times the crystal was irreversibly damaged.
Similar observations were made in a model experiment with a soap bubble crystal. After destroying a sufficient number of bubbles of a two-dimensional, hexagonally ordered raft of soap bubbles, the distorted crystal quickly reorderes by shifting the vacancies to the border of the crystal.Thermal activation plays no role for the movement of the macroscopic bubbles.
These mass transport processes resemble very much the avalanches occuring in a pile of sand or snow, where slight distortions drive the system unstable. The subsequent 'mass slide' significantly lowers the energy, before the system locks into a metastable configuration again. However, in these latter systems no better degree of order is achieved by the mass transport.
The stress-driven formation of order without diffusive mass transport might have particular consequences not only for the crystallization of proteins. It might be of particular relevance also for the epitaxial growth of crystalline material, important for thin layer technology and semiconductor fabrication.
Nature