¹³ C NMR studies of insulin. Part I-Spectral assignments

Natural abundance 75 MHz ¹³ C NMR spectral assignments are reported for bovine and porcine zinc insulin in solution. A large number of protein resonances are well resolved, and approximately 80% of these have been assigned to either residue types or to specific sites within the protein. Assignment techniques included consideration of free amino acid or peptide shifts pH studies and comparison of sequence and spectral differences between bovine and porcine insulin, in addition to the use of NMR relaxation times. The DEPT spectral editing technique was also found to be particularly valuable as an assignment aid. This technique allows subspectra containing only CH, CH₂ or CH₃ carbon types to be generated. The method also produces signal enhancement relative to broad band decoupled ¹³ C NMR spectra of large proteins which generally have reduced nuclear Overhauser enhancements.