Two-dimensional 1H NMR of two chemically modified analogs of the basic pancreatic trypsin inhibitor. Sequence-specific resonance assignments and sequence location of conformation changes relative to the native protein.

Two-dimensional nuclear magnetic resonance was used to obtain sequence specific assignments for the 1H NMR spectra of two chemically modified analogs of the basic pancreatic trypsin inhibitor. In one analog the disulfide bond 14-38 was cleaved, in the second derivative the N-terminus was transaminated. From measurements of the chemical shifts and determination of the sequence locations of slowly exchanging backbone amide protons it was found that conformational differences between the native inhibitor and the chemical modifications occur exclusively near the modification sites and that the internal hydrogen bonds are nearly fully preserved. Intriguing conformation differences with respect to the native protein are that for five residues in the transaminated inhibitor and for one residue in the reduced inhibitor multiple local conformers are indicated, and that the four internal water molecules observed in the crystal structure of the native inhibitor appear not to be preserved after reduction of the disulfide bond 14-38.