A hydrogen bond is a weak attraction between a slightly positive (δ+) hydrogen atom that is covalently bonded to an electronegative atom (O or N) and a slightly negative (δ−) O or N atom nearby. In proteins, hydrogen bonds are one of the main forces that hold the chain in its folded shape.In the polypeptide backbone. Every peptide bond contains a C=O group (the O is δ−) and an N–H group (the H is δ+). Hydrogen bonds between these groups in different parts of the chain produce the secondary structure:• In an α-helix, the chain coils, and the C=O of each amino acid forms a hydrogen bond with the N–H of the amino acid four places further along.
• In a β-pleated sheet, sections of the chain lie side by side, and hydrogen bonds form between the C=O and N–H groups of neighbouring sections.
C=Oδ− ··· δ+H–N Between R groups. Polar R groups containing –OH or –NH2 groups can also form hydrogen bonds with each other, helping to hold the tertiary structure.Weak but many. A single hydrogen bond is weak, but a protein contains hundreds of them, so together they give a stable shape. Because each is weak, they are easily broken by high temperature (increased vibration) or changes in pH. When enough are broken the protein loses its shape and is denatured.Worked example: in an α-helix the C=O of amino acid 3 bonds to the N–H of amino acid 3 + 4 = 7.
• In a β-pleated sheet, sections of the chain lie side by side, and hydrogen bonds form between the C=O and N–H groups of neighbouring sections.