Proteins are the key building blocks of all biological matter. While engineers predominantly use concrete, metals, ceramics and synthetic polymers as structural materials for their high strength and durability, Nature exploits complex mechanical and chemical features of proteins for building strong, elastic and robust materials and structures. For example, spider silk, amyloid (sturdy protein fibrils found in many diseases) and muscle fibers are made entirely of proteins and blend extensibility with high strength to achive extreme toughness. These super-fibers represent an alternative scheme of material design to biomineralization, which allows for incorporation of minerals in protein scaffolds to build very stiff and tough materials such as nacre, mollusk shells and bone. Many scientists have been intrigued by Nature's unknown recipe for creating soft yet durable and strong materials. These materials are different from their synthetic equivalents because they employ hydrogen bonds that are much weaker than covalent or metallic bonds, and exhibit entropic elasticity at the nano-scale.

Protein materials such as spider silk have a modular, hierarchical architecture that extends from nano to macro. This leads to very complex mechanical behavior at macro-scale which can't easily be described by simple continuum formulations. Early attempts in the field to explain elasticity of biopolymers such as silk illustrated the need to take into account the micro-structural features of the material for describing the mechanical behavior. The first paper by Yves Termonia [1] is one of the early modeling attempts of spider silk elasticity using structural evidence from X-ray diffraction studies. A key finding from this paper is that the size of beta-sheet nanocrystals in spider silk influences elastic behavior and the observed structures in silk are optimal from a mechanical perspective.

The second paper [2] is a recent review on the mechanical characterization of individual proteins using experimental techniques (for instance Atomic Force Microscopy [AFM]) and computational approaches (for instance Steered Molecular Dynamics [SMD] simulations). The studies reviewed in this work explore mechanical functions of proteins at the nano-scale and molecular mechanisms of the elasticity of extracellular matrix and muscle proteins. Nanomechanical characterization of proteins by computational and experimental approaches is a crucial step in building multi-scale models that can accurately predict macro-scale behavior of biological materials. Therefore, scientists working in this field generally need to appreciate both continuum approaches (constitutive relations, FEM studies etc.) as well as biophysical studies (single protein manipulation, statistical mechanics).

The last paper [3] is a very recent study on amyloids, protein materials that were subject to a tremendous amount of research in the past decade due to their association with many diseases. Amyloid protein structures constitute an intriguing class of protein materials that self-assemble at room temperature into very stable and structured fibrils. Amyloid deposits in tissue are known to be linked to pathological conditions that occur in diseases such as Alzheimer's, Parkinson's, prion diseases or type II diabetes. Amyloids also have intriguing properties that resemble spider silk, and are found in many natural adhesives. The AFM study referenced here is one of the few experiments that investigated the elasticity and fracture behavior of amyloids. Along with other studies, this paper shows that the intriguing mechanical properties (e.g. elastic modulus comparable to spider silk) of amyloid fibrils derive from generic interbackbone hydrogen-bonding network common to all amyloids.

These are only a few examples of the many inspirational and exciting works that study protein materials from a mechanical perspective. The three papers included here are not nearly enough to cover all aspects of the rapidly developing field of protein mechanics, but will hopefully illustrate promising directions in the field and promote some discussion regarding elasticity, deformation and fracture mechanics of protein structures and materials.

 REFERENCES:

1. Termonia, Y., Molecular Modeling of Spider Silk Elasticity. Macromolecules, 1994. 27(25): p. 7378-7381.

2. Sotomayor, M. and K. Schulten, Single-molecule experiments in vitro and in silico. Science, 2007. 316(5828): p. 1144-1148.

3. Knowles, T.P., et al., Role of intermolecular forces in defining material properties of protein nanofibrils. Science, 2007. 318(5858): p. 1900-1903.