Salt Bridges of protein
A salt bridge is defined as a hydrogen bonded pair of charges of opposite sign. Obviously, the formation of a salt bridge depends on the protonation state of the partners and hence on pH.
Salt bridges are interactions of amino acids with opposite charge where at least two heavy atoms lie within a hydrogen bonding distance. Often found in solvent exposed parts of proteins, they are susceptible to external interactions, primarily with water.
In a salt bridge, a proton migrates from a carboxylic acid group to a primary amine or to the guanidine group in Arg. Typical salt bridges involve Lys or Arg as the bases and Asp or Glu as the acids. Of all the non-covalent interactions, salt bridges are among the strongest.
Salt bridges contribute to protein structure and to the specificity of interaction of proteins with other biomolecules, but in doing so they need not necessarily increase a protein's free energy of unfolding.
The net electrostatic free energy of a salt bridge can be partitioned into three components: charge–charge interactions, interactions of charges with permanent dipoles, and desolvation of charges.
