RING-CURRENT EFFECTS AND MAGNETIC-ANISOTROPY EFFECTS OF CARBONYL GROUPS ON THE ALPHA-CH PROTON CHEMICAL-SHIFTS OF THE BASIC PANCREATIC TRYPSIN-INHIBITOR AND TENDAMISTAT

The α-CH proton chemical shifts of basic pancreatic trypsin inhibitor (BPTI) and tendamistat were calculated by taking into account both the ring-current shielding effect from aromatic rings and the magnetic anisotropy shielding effect from all carbonyl bonds on the basis of the atomic coordinates reported with X-ray diffraction analysis. The ring-current shielding effects were evaluated according to the Johnson-Bovey equation. The magnetic anisotropy shielding effects were evaluated according to the ApSimon equation using the susceptibility anisotropies of the carbonyl bond reported by Ziircher with the dipole origin of carbonyl bond located 1.0 Å away from the carbon atom. The agreement between the calculated and observed α-CH chemical shifts is good for both proteins. The α-CH chemical shifts of the residues of BPTI in the β-sheet region are well interpreted by the magnetic anisotropy effects of the carbonyl groups, and the remarkably high field shift of the α-CH proton of Cys-51 is due to the ring-current shielding effect from the phenyl ring of the Phe-45 residue. In addition, most of the a-CH chemical shifts for tendamistat are determined by the magnetic anisotropy effects of the carbonyl group. © 1991.