In proteins, the α helix is a major structural motif in secondary structure. It was first postulated by Linus Pauling, Robert Corey, and Herman Branson in 1951 based on the known crystal structures of amino acids and peptides and Pauling's prediction of planar peptide bonds. The amino acids in an α helix are arranged in a helical structure, about 5 Å wide. Each amino acid means a 100° turn in the helix, and the α-C atoms are only 1.5Å apart. The helix is tightly packed, there is almost no free space within the helix. All amino acid side-chains are arranged at the outside of the helix. The N-H group of amino acid (n) can establish a hydrogen bond with the C=O group of amino acid (n+4). short polypeptides usually are not able to adopt the alpha helical structure, since the entropic cost associated with the folding of the polypeptidic chain is too high. Some amino acids (helix breakers) like proline will disrupt the helical structure. α helices are one of the basic structural elements in proteins, together with beta-sheets.
Reference: David Eisenberg, The discovery of the α-helix and β-sheet, the principal structural features of proteins. Proceedings of the National Academies of Sciences USA. (2003). 100: 11207-11210. http://www.pnas.org/cgi/content/full/100/20/11207