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Revised Manuscript Received August 12, 1993® abstract: Hydrophobic residues in the core of a truncated formof chymotrypsin inhibitor 2 (CI2) have been mutated in order to measure their contribution to the stability of the protein. The free energy of unfolding of wild-type and mutants was measured by bothguanidinium chloride-induced denaturation and differential scanning calorimetry. The two methods give results for the changes in free energy on mutation that agree to within 1% or 2%. The average change in the free energy of unfolding (±standard deviation) for an lie—*• Val mutation is 1.2±0.1 kcal mol-1, for a Val-*• Ala mutation 3.4±1.5 kcal mol-1, and for either an He-» Ala or a Leu-* Ala mutation 3.6±0.6 kcal mol-1. This gives an average change in the free energy of unfolding for deleting one methylene group of 1.3±0.5 kcal mol-1. Twosignificant correlations were found between the change in the free energy …