Energy landscape and fluctuations in proteins

Complex systems such as biomolecules or glasses do not exist in a unique structure, but can assume a very large number of somewhat different conformations. The organization of the conformations can be described in the energy (or conformation) landscape, where individual valleys are called conformational substates. Proteins are systems where the energy landscape has been studied in some detail. Their landscape is organized in a hierarchy that contains three types of substates, taxonomic, statistical, and few‐level. Conformational substates are found in a wide variety of proteins. Some of their characteristics are described and the open experimental and theoretical problems are listed.