The salt bridge that occur between residues i and i + 4 in helices has long been thought th o be important in thermostability. Experiments on several proteins and model systems have shown that a single solvent exposed slat bridge contributes 0.1-0.5kcal/mol to stability.That is relatively low. The reason of it is partly a high apparent dielectric constant on the surface of a protein can reduce the electrostatic component of salt bridge.