Proton magnetic resonance studies of actinomycin‐related peptides containing N‐methyl groups

The PMR spectra of several cyclic and acyclic dipeptide derivatives representing portions of the actinomycin structure have been studied. In the spectra of the acyclic compounds temperature‐dependent duplicity resulted from the equilibration of conformers possessing cis and trans peptide bonds. The lanthanide shift reagent Eu(FOD) 3 was utilized to distinguish N‐methyl groups in the two conformations and observations were made on the steric dependence of the observed shifts. The origin of the wide variation in chemical shift of N‐methylvalyl α‐protons is discussed in relation to the conformation of actinomycin.