Entropy Drives the Formation of Salt Bridges in the Protein GB3

Salt bridges are very common in proteins. But what drives the formation of protein salt bridges is not clear. In this work, we determined the strength of four salt bridges in the protein GB3 by measuring the ΔpK_a values of the basic residues that constitute the salt bridges with a highly accurate NMR titration method at different temperatures. The results show that the ΔpK_a values increase with temperature, thus indicating that the salt bridges are stronger at higher temperatures. Fitting of ΔpK_a values to the van't Hoff equation yields positive ΔH and ΔS values, thus indicating that entropy drives salt-bridge formation. Molecular dynamics simulations show that the protein and solvent make opposite contributions to ΔH and ΔS. Specifically, the enthalpic gain contributed from the protein is more than offset by the enthalpic loss contributed from the solvent, whereas the entropic gain originates from the desolvation effect.