The alpha-helix ([latex]\alpha[/latex]-helix) is a common secondary structure motif in proteins. It is a right-handed coiled conformation in which every backbone N-H group donates a hydrogen bond to the backbone C=O group of the amino acid located four residues earlier ([latex]i+4 \rightarrow i[/latex] hydrogen bonding). This regular pattern pulls the polypeptide chain into a helical shape.
Linus Pauling and his colleagues predicted the existence of the alpha-helix in 1951 before it was experimentally observed in detail by Max Perutz in the structure of myoglobin. Their prediction was based on a deep understanding of the planar nature of the peptide bond and the principles of hydrogen bonding. They meticulously built physical models to find stable conformations of a polypeptide chain that maximized hydrogen bonding while respecting steric constraints.
The alpha-helix has specific geometric properties: 3.6 residues per turn, a pitch of 5.4 Å (0.54 nm), and backbone dihedral angles ([latex]\phi, \psi[/latex]) typically around (-60°, -45°). The side chains of the amino acids extend outwards from the helical axis, allowing them to interact with the environment or other parts of the protein. Certain amino acids, like alanine, leucine, and methionine, are considered ‘helix formers,’ while others, like proline (which lacks a backbone N-H for hydrogen bonding and has a rigid ring) and glycine (which is too flexible), are ‘helix breakers.’ The discovery of the alpha-helix and the beta-sheet provided the first fundamental insights into how a simple linear chain of amino acids could fold into complex, stable, and functional three-dimensional structures.
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