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Secondary structure

Whilst peptides do not always exhibit ordered three-dimensional structure, most proteins consistently fold into the same configuration. The three-dimensional structure of proteins is uniquely determined by its primary structure[Anfinsen, 1973], although the folding process can be aided by molecules known as chaperones[Hartl, 1996]. The first experimentally determined protein structure, the X-ray diffraction structure of myoglobin by Kendrewkendrew:myoglobin1, showed the polypeptide chain bundled into a compact tertiary structure. For an introduction to protein structure determination, see Chapter 17 in Branden and Toozebranden:tooze. Subsequent higher resolution structures of myoglobin[Kendrew et al., 1960] confirmed the existence of a regular helical arrangement, called $\alpha$-helix, in much of the polypeptide chain, proposed by Pauling and Corey[Pauling et al., 1951] from theoretical studies of peptides and X-ray analysis of hair fibres. At that time, Pauling and Coreypauling:pnas1 had also correctly predicted the existence of sheet-like structures of non-covalently cross-linked strands of extended polypeptide chain which they called $\beta$-sheet. $\alpha$-helices and $\beta$-sheets are the most common form of local regular structure, known as secondary structure, in proteins.