Analysis of immunoglobulin domain interactions. Evidence for a dominant role of salt bridges

Available crystallographic data for homologous immunoglobulin constant domains were correlated with measured association constants for these domains. High correlation was found between the association constant and both the buried surface area (number of interdomain contacts) and the number of salt bridges formed in the interaction, whereas no correlation with the number of hydrogen bonds between domains was evident. The total free energy of binding, as determined from the association constant, was related to the number of contacts, hydrogen bonds and salt bridges found in the domain:domain interface by the crystallographic studies. These calculations yielded reasonable average energy terms for each interaction category.