ionic interaction

An ionic interaction is the simple attraction between opposite charges, just like a positive and a negative magnet pulling together. In a binding site, a positively charged group on the drug is drawn to a negatively charged group on the target, or vice versa. Of all the noncovalent forces it is one of the strongest in principle, reaching far across the pocket before short-range contacts even matter.

The strength of this Coulombic attraction depends heavily on the surroundings. In a vacuum it would be enormous, but water and the protein's own polar groups screen the charges, weakening the pull dramatically. The local dielectric environment — whether the charges sit exposed to water or buried in a greasy pocket — can make the same pair of charges worth a little or a lot.

In practice, ionic groups are double-edged. They can deliver potency and steer selectivity toward a charged residue, but a permanent charge or strongly ionizable group often hurts membrane permeability and oral absorption. Medicinal chemists frequently tune the pKa of basic or acidic groups to balance binding against the need for the molecule to cross membranes.