Entropic effects of disulphide bonds on protein stability.

To measure the thermodynamic consequences of the reduction in the number of polypeptide-chain conformations that accompanies protein folding, we developed a method called loop permutation analysis. In this approach, the stabilizing contributions of three engineered disulphide bonds were compared in extended and circularly permutated mutants of phage T4 lysozyme. The observed differences in disulphide contributions, although qualitatively consistent with theoretical estimates, were not solely proportional to the differences in loop length. These findings suggest that in addition to the length of the chain, the polypeptide sequence may influence the energetic consequences of conformational restrictions.