Salt linkages electrostatic/ionic

Salt-linkages (electrostatic forces ionic bonds)... 

Most often, the electrostatic bond is between two ions (the ionic bond or salt linkage ). There are variants in which the bond is formed between an ion and a dipole or between two dipoles. They are all maintained by purely electrostatic forces.The ionic bond has a strength of about 5 kcal/mol and declines by the second power of the distance between the opposite charges. Sodium chloride (Na Cl ) is a typical example. In aqueous solution, each ion is able to move about freely so long as it does not leave the field of its counterion in other words, the bond is non-directional and non-rigid. 

Salt linkages (ionic or electrostatic bonds) result from the interaction of the positively charged side chains of lysine, arginine or to a lesser extent of histidine, and the negatively charged carboxyl groups of the side chains of glutamic and aspartic acids. 

Electrostatic bonds—also called an ionic bond, salt linkage, salt bridge, or ion pair— arise from the attractive force (about 3-7 kcal/mol) between oppositely charged R groups on the enzyme. This force decreases with an inorease in distance between the positive and negative groups and is optimal in water at a distance of 2.8 A. 

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