The Influence of Localized Chemical Modifications of the Basic Pancreatic Trypsin Inhibitor on Static and Dynamic Aspects of the Molecular Conformation in Solution

High‐resolution 1H nuclear magnetic resonance was used to characterize the molecular conformations in solution of the basic pancreatic trypsin inhibitor and three chemical modifications of this protein. It was found that, except for local variations near the modification site, the average spatial structure was preserved after reduction of the disulfide bond 14–38, after cleavage of the reactive peptide bond Lys‐15–Ala‐16, and after removal of the residues Ala‐16 and Arg‐17 from the reactive‐site‐cleaved inhibitor. In all four species the average solution conformation corresponds closely to the crystal structure of the native inhibitor. The stability of the globular conformation in the modified proteins is, however, markedly reduced, i.e. the denaturation temperature is lowered and the interior amide protons exchange more rapidly with the solvent. In contrast, the internal mobility of the aromatic rings is essentially unaffected by these modifications of the covalent structure. These observations and inspection of the molecular structure of the native inhibitor provide direct evidence for the occurrence of hydrophobic stability domains within these relatively small proteins. Functional roles of hydrophobic clusters in the architecture of globular proteins are discussed.