Actinomycin D, 1H NMR studies on intramolecular interactions and on the planarity of the chromophore.

The conformation of actinomycin D in acetone and chloroform solution at different temperatures has been studied by 1H NMR spectroscopy. At lower temperature the resonances due to the two chromophoric amino protons were observed. These signals exhibit very different resonance positions indicating a severely hindered rotation of the 2-amino group and the presence of a hydrogen-bond connecting the 2-amino and the 1-carbonyl groups. In 1H NMR spectra of partially 15N-enriched actinomycin D, the 1JN-H coupling constants at the 2-amino group were determined and a strong sp2 character for the 2-amino nitrogen was deduced. The strong amide character of the 2-amino group is caused by mesomerism involving the 1-carbonyl group. The amino proton signals are sensitive indicators for differences in the spatial relationship of the diverse parts of the actinomycin molecule. At lower temperatures a simultaneous and selective broadening of the alpha ring threonine and valine amide proton signals as well as of the 2-amino group resonance was observed, indicating the presence of one dynamic process in the molecule which slows down upon temperature reduction. A swinging motion of the N(10) nitrogen through the chromophore plane would explain this observation. The interpretation of these results requires the presence of a non-planar chromophoric system in the actinomycin molecule in acetone and chloroform solution. The possible implications of this non-planarity for the intercalation process and for the biological activity of the drug are discussed.