Elements of Protein Structure

Hydrogen Bonds

The hydrogen bond (H-bond) is a SPECIAL CASE of polar interaction. While it is called a "hydrogen bond", it is NOT a covalent bond. Like any other polar interaction it is between two separate molecules. Recall from the previous page that a polar interaction was ANY interaction between two polar molecules. The H-bond, while it also requires favorable interaction between two polar molecules also includes specific rules about:

Depicted are the sidechains of a threonine and a serine. Note the orientation of the "O-H" bond of the serine relative to the "O" atom of th threonine.

The hydrogen bond involves neither a group whose proton can dissociate nor is it covalent

Note: in the two examples presented on this page neither involve a group that is readily ionized at physiological pH.
Nomeclature of H-bond Two Peptide bonds are represted. Note how the "N-H" bond of the "bottom" group is pointing right at the Oxygen atom of another peptide bond.In this picture which is the H-bond donor and which is the H-bond acceptor? Represetnted here are two peptide bonds. The blue nitrogen of the "lower" amide very close to the red oxygen of the "upper" amide. It is easy to see the N-H bond points directly at the "O" of the neighboring amide.

Recall that the amide bond (peptide bond) is also polar! H-bonds between peptide bonds are very common (See helices and sheets)