Mutations in the complementarity-determining regions do not cause differences in free energy during the process of formation of the activated complex between an antibody and the corresponding protein antigen.

Effects of mutations on antigen-antibody interactions were energetically examined using several kinds of Fv fragments, derivatives of a monoclonal antibody, D1.3, that is specific for hen egg-white lysozyme. According to the transition-state theory, the thermodynamic parameters involved in pre and post-processes of formation of the activated complex were estimated separately. In the pre-process, there is a strong compensation between changes in entropy and changes in enthalpy caused by substitutions of amino acid residues. Thus, the differences in the association constant (KA) caused by mutations are not related to the pre-process but to the post-process.