Spectral assignment and conformational analysis of cyclic peptides by carbon-13 nuclear magnetic resonance.

Tentative assignment for each carbon resonance in the natural abundance high resolution carbon-13 nuclear magnetic resonance Fourier transform spectrum at 25.2 MHz of the biologically active cyclic octapeptides, synthetic oxytocin (43 carbons) and lysine-vasopressin (46 carbons) and the cyclic dodecapeptide bacitracin (66 carbons) has been made using a number of different techniques. The 1% assignments of the constituent amino acids were used to investigate the conformation of these cyclic peptides in aqueous solution. The chemical shift nonequivalence found for the end chain and P-substituted methyl carbons of the 3 isoleucyl residues in bacitracin may result from different chemical environments imposed by the conformation of this peptide. Furthermore, the small differences (less than 2 ppm) in chemical shifts of the cyclic peptide 13C resonances relative to the amino acjds (correcting for internal peptide bond, Nand C-terminal, and pH effects) and linear peptides may also reflect the extent that 13C shifts are affected by different conformations.