Noncovalent interactions are very common in macromolecules
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Introduction
Noncovalent interactions are very common in macromolecules (including proteins), due to the relative weakness of these interactions.
Indeed, whereas covalent bonds require an energy input of 65 to 175 kcal/mol to be broken, the energy content of a noncovalent bond is only up to a few kcal/mol.
The weakness of these interactions allows proteins to change conformation and bind ligands, two properties that are highly important for their function.
Although they are weak, the high number of noncovalent interactions still allows them to stabilize three-dimensional protein structures.
The nature and strength of noncovalent interactions in a protein are affected by the chemical nature of the protein’s environment.
It is therefore customary when discussing protein structure to take into consideration not only the protein and any ligand it may bind (both are referred to as the ‘solute’), but also the molecules constituting their environment (referred to as the ‘solvent’).
There are two main types of noncovalent interactions:
Electrostatic
Electrostatic interactions occur between electrically charged atoms, and include both attractive and repulsive forces.
The charges may be full or partial, and partial charges may be fixed or induced.
One class of electrostatic interactions, van der Waals interactions, occur between induced partial charges. This type of interaction has several unique features (e.g., it occurs between any pair of atoms that are close enough to each other).
Nonpolar
These attractive interactions, resulting from the hydrophobic effect, are most noticeable between atoms or chemical groups that are devoid of charge.