The alpha-helix

The peptide bond has a hydrogen bond donor (NH) and a hydrogen bond acceptor (C=O). As a protein folds, many hydrogen bonds to water are lost.  Hydrogen bonds between peptide groups preserve H-bonding of the peptide backbone in the hydrophobic environment of the protein core.  Repeating peptide H-bonds can generate regular structures (alpha-helix, beta-sheet, beta-turn) known as secondary structure.

The wireframe model to the left shows all the nonhydrogen atoms comprising the backbone of a helical 15-mer polypeptide segment. All the carbonyl groups point toward the C-terminus.

  Within an alpha-helix, each carbonyl O atom (residue n) is hydrogen bonded to the backbone amide N of the fourth residue further toward the C-terminus of the helix (residue n + 4).  All the hydrogen bonds point in the same direction and are nearly parallel to the helix axis.

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