After observing the simultaneous rupture of these hydrogen-bond clusters within the proteins in their atomistic simulations, Buehler and Keten wanted to know why the bonds break in small clusters, even in long strands with many hydrogen bonds. They used the laws of thermodynamics to explain this phenomenon. The paper in Nano Letters describes how the external force changes the entropic energy in the system and leads to the rupture of hydrogen bonds. By calculating the energy necessary to initiate the unfolding process in a protein molecule, they demonstrated that adding more hydrogen bonds in longer strands would not increase the materials strength.
You would simply have this long chain of beta strands with lazy bonds that dont contribute to the strength of the assembly, said Keten. But a material that employs many short beta strands folded and connected by three or four hydrogen bonds may exhibit strength greater than steel. In metals, the energy would be stored directly in much stronger bonds, called metallic bonds, until bonds rupture one by one. In proteins, things are more complicated due to the entropic elasticity of the noodle-like chains and the cooperative nature of the hydrogen bonds.
Structural proteins contain a preponderance of beta-sheets, sections that fold in such a way that they look a bit like old-fashioned ribbon candy: short waves or strands appear to be stacked on top of one another, each just the right length to allow three or four hydrogen bonds to connect it to the section above and beneath.
Beta sheets with short strand lengths connected by three or
|Contact: Denise Brehm|
Massachusetts Institute of Technology, Department of Civil and Environmental Engineering