Solids possess a periodic, glasses and proteins a non-periodic spatial structure. While the disorder in glasses is random, it has been carefully selected by evolution in proteins. Non-periodicity consequently describes the situation in a protein better than disorder. In contrast, the weak bonds in a biomolecule can be broken by thermal fluctuations. A biomolecule can therefore execute large motions, it can breathe and can act as a miniature machine.
Solids are spatially homogeneous, apart from surface effects and from defects.
In glasses, inhomogeneities are random and minor. Through genetic engineering, the primary sequence is modified at the desired location and this modification leads to the corresponding change in the protein. The difference between a crystalline solid on the one hand, glasses, spinglasses, and biomolecules on the other hand, can be expressed in another way. The energy surface of a crystal, that is its energy as a function of its conformation, is non-degenerate and has a single minimum as shown in Fig. The energy hypersurface then is a function of all of these coordinates.