The alpha-helix is a spiral secondary structure of proteins or polypeptide chains which is stabilized by hydrogen bonds.
The alpha-helix is a right-handed coiled spiral with an average of 3.6 amino acids per turn. The pitch is at a distance of 5.4 Å (0.54nm). Combining the average of amino acids per turn and the distance of the pitch a distance between each amino acid of 1.5 Å (0.15nm) is estimated. The hydrogen bonds are estabished between a carboxyl group of the an amino acid and the amino group of the fourth subsequent amino acid. The distance between these two hydrogen bond forming amino acids is 2.9 Å (0.29nm). The hydrogen bonds are approximately parallel to the axis of the alpha-helical molecule. The radicals of the alpha-helix forming amino acids are directed outward from the axis of the alpha-helix.
Moreover it is possible to make a prediction whether an amino acid containing molecule will form an alpha-helix or not. In the presence of high amounts of "helix formers" which are f.e. glutamic acid, leucine, alanine, histidine and methionine there is a high probability of an alpha-helix formation. However, other amino acids like glycine, proline and tyrosine destabilize the alpha helical structure.
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