ionic (electrostatic) interaction

Bring a positive and a negative magnet near each other and they snap together; reverse one and they shove apart. Charged parts of molecules behave the same way. An ionic interaction is simply the attraction between a full positive charge and a full negative charge — opposite charges pulling together across a gap.

Some groups on biological molecules carry whole electrical charges at the cell's pH: a deprotonated carboxyl group is negative (it lost an H+), a protonated amino group is positive (it gained one). When a plus group and a minus group sit near each other, they attract through a plain electrostatic force, sometimes called a salt bridge when it forms inside or between proteins. This attraction is stronger than a hydrogen bond and reaches across longer distances, but in the watery cell it is heavily softened: surrounding water molecules and dissolved salt ions crowd in and shield the charges, so the pull is much weaker than it would be in dry air. Charges of the same sign, of course, repel.

Ionic interactions show up wherever charge meets charge. They help hold a folded protein together, clamp positively charged proteins onto the negatively charged backbone of DNA (which is studded with negative phosphate groups), and let charged drugs dock into their targets. Because water and salt shield them, their strength depends on the salt concentration — which is why biologists adjust salt in their buffers to tune how tightly molecules stick. Add enough salt and you can pry apart a protein from the DNA it grips.