<P> Once translated by a ribosome, each polypeptide folds into its characteristic three - dimensional structure from a random coil . Since the fold is maintained by a network of interactions between amino acids in the polypeptide, the native state of the protein chain is determined by the amino acid sequence (Anfinsen's dogma). </P> <P> Around 90% of the protein structures available in the Protein Data Bank have been determined by X-ray crystallography . This method allows one to measure the three - dimensional (3 - D) density distribution of electrons in the protein, in the crystallized state, and thereby infer the 3 - D coordinates of all the atoms to be determined to a certain resolution . Roughly 9% of the known protein structures have been obtained by nuclear magnetic resonance techniques . The secondary structure composition can be determined via circular dichroism . Vibrational spectroscopy can also be used to characterize the conformation of peptides, polypeptides, and proteins . Two - dimensional infrared spectroscopy has become a valuable method to investigate the structures of flexible peptides and proteins that cannot be studied with other methods . Cryo - electron microscopy has recently become a means of determining protein structures to high resolution, less than 5 ångströms or 0.5 nanometer, and is anticipated to increase in power as a tool for high resolution work in the next decade . This technique is still a valuable resource for researchers working with very large protein complexes such as virus coat proteins and amyloid fibers . A more qualitative picture of protein structure is often obtained by proteolysis, which is also useful to screen for more crystallizable protein samples . Novel implementations of this approach, including fast parallel proteolysis (FASTpp), can probe the structured fraction and its stability without the need for purification . </P> <P> Proteins are often thought of as relatively stable structures that have a set tertiary structure and experience conformational changes as a result of being modified by other proteins or as part of enzymatic activity . However proteins have varying degrees of stability and some of the less stable variants are intrinsically disordered proteins . These proteins exist and function in a relatively' disordered' state lacking a stable tertiary structure . As a result, they are difficult to describe in a standard protein structure model that was designed for proteins with a fixed tertiary structure . Conformational ensembles have been devised as a way to provide a more accurate and' dynamic' representation of the conformational state of intrinsically disordered proteins . Conformational ensembles function by attempting to represent the various conformations of intrinsically disordered proteins within an ensemble file (the type found at the Protein Ensemble Database). </P> <P> Protein ensemble files are a representation of a protein that can be considered to have a flexible structure . Creating these files requires determining which of the various theoretically possible protein conformations actually exist . One approach is to apply computational algorithms to the protein data in order to try to determine the most likely set of conformations for an ensemble file . </P>

Which of the following levels of protein structure describes a linear sequence of amino acids