 <unk> the tertiary structure of a protein , or the <unk> structure of its complexes , can provide important clues about how the protein performs its function . Common experimental methods of structure determination include X @-@ ray crystallography and NMR spectroscopy , both of which can produce information at atomic resolution . However , NMR experiments are able to provide information from which a subset of distances between pairs of atoms can be estimated , and the final possible conformations for a protein are determined by solving a distance geometry problem . Dual <unk> <unk> is a <unk> analytical method for measuring the overall protein conformation and conformational changes due to interactions or other stimulus . <unk> <unk> is another laboratory technique for determining internal β @-@ sheet / α @-@ helical composition of proteins . <unk> microscopy is used to produce lower @-@ resolution structural information about very large protein complexes , including assembled viruses ; a variant known as electron crystallography can also produce high @-@ resolution information in some cases , especially for two @-@ dimensional crystals of membrane proteins . <unk> structures are usually deposited in the Protein Data Bank ( PDB ) , a freely available resource from which structural data about thousands of proteins can be obtained in the form of <unk> coordinates for each atom in the protein . 
