Chemistry and Biology of Proteins - F. Haurowitz 1953

Internal Structure of Globular Proteins
Salt and Hydrogen Bonds in Proteins

As is evident from the data presented in the preceding sections, peptide chain branching and cyclization play only a minor role in the Introduction/12.html">Structure of Globular Proteins. The folding of long peptide chains is presumably driven not by strong chemical bonds, but by weaker forces arising from the mutual attraction of ionic and polar groups. It has been emphasized previously that proteins contain both positively and negatively charged groups. Oppositely charged groups attract one another through electrostatic forces. Similarly, dipoles will attract each other As a result of dipole association (see Fig. 22). The mutual attraction of ionic groups varies inversely as r² (where r is the distance between the ionic groups), whereas the binding capacity of dipoles varies inversely as r⁶ or r⁷. This means that dipole-dipole forces are effective only at very short ranges. Consequently, bonding via dipoles can occur only when they are in close contact with one another [95].

The electrostatic forces responsible for bonding between ions and dipoles also drive The formation of so-called Hydrogen Bonds [96, 97]. The majority of hydrogen bonds in proteins are formed between the imino groups of peptide bonds in one chain and the carbonyl groups of peptide bonds in another chain:

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The structure of a Hydrogen bond is illustrated by the following electronic formula: