Ionic bond

The ionic bond, also called salt bridge, it is, perhaps, the easiest non-covalent bond type to understand. As the name implies, it is an interaction that occurs between ions, i.e., between positive and negative charges. In biochemistry, the concept of ion is slightly different, because in a biomolecule (a protein, for example) we can have multiple regions with negative and/or positive charges. For this to happen just it is only necessary the presence of ionizable functional groups...

Therefore, when one biomolecule has a region with a negative charge (a phosphoryl or carboxyl group, for example), it can establish electrostatic interactions with a region of a biomolecule that presents a positive charge (e.g., amine or imidazole functional groups). This electrostatic attraction that occurs between the opposite charges is the ionic bond.

It is a type of interaction that occurs between polar molecules or at least between ionizable polar regions of biomolecules. As examples, we have the case of an interaction between a lysine and a glutamate in the interior of a protein or the interaction between DNA and histones, among many others...