The folding is so compact that the density of a protein, for instance, is nearly the same as the density of a crystal made out of one amino acid. Indeed, proteins and solids have a number of similar properties. Proteins have, however, also features that distinguish them from solids. In a sense, they can be considered a separate state of matter. Since these features are crucial for the functioning of proteins, we compare biomolecules, glasses, spinglasses, and solids.
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.