13C NMR study of the effects of mutation on the tryptophan dynamics in chymotrypsin inhibitor 2: correlations with structure and stability.

Recombinant chymotrypsin inhibitor 2 (CI-2) and the three mutants Ile39-->Val, Ile39-->Leu, and Arg67-->Ala were successfully enriched with [2-13C]tryptophan at position 24 within the hydrophobic core of the protein. Carbon-13 NMR relaxation measurements were then used to investigate the effect of these mutations on the dynamics of the tryptophan residue. In addition, the stability of wild-type and mutant CI-2s was measured by their susceptibility to unfolding by guanidine hydrochloride. The mutant proteins were all found to be less stable, giving delta delta GU values relative to wild-type of 1.17, 1.96, and 1.21 kcal mol-1, respectively. The indole moiety of the tryptophan residue was found to be more mobile in all the mutants studied than in wild-type CI-2. Order parameters of 0.69, 0.60, 0.56, and 0.44 were derived for wild-type, Ile39-->Val, Ile39-->Leu, and Arg67-->Ala CI-2, respectively. It is concluded that there is a correlation between the protein stability and the picosecond dynamics within the hydrophobic core and that mutations can influence the dynamic behavior of the residues that are relatively distant in the three-dimensional structure.