Salt bridges

There are many ways of saying that salt bridges are good for the stability of a protein. One way is plus and minus attracts each other, and when you bring them together, you need to exert a force to separate them again in the unfolding process. Another way is saying that it costs energy to maintain a charge. If you neutralize charges, as you do partly by forming a salt bridge, you gain energy which is good for protein stability. However you formulate it, salt bridges are good for protein stability.

Figure 33. Example of a salt bridge. There are two schools. One school calls it a salt bridge when the two residues make a charged hydrogen bond. The other school also calls plus - minus interactions a salt bridge when the charged groups are quite far away from each other, for example up to 8 or even 12 Ångström.

If you look at the coarse introduction to protein energetics and force fields (in the supplemental material), you see that Van der Waals interactions or neutral non-bonded interactions depend on the distance as a function of R6 at least. But charge-charge interaction carry much further through space. In energy calculations one can typically stop looking at salt bridges once the charged atoms are more than 10 Ångström  away from each other, but there are strong indications that that is a too short cutoff.

Question 23: