Solution Structure of [Me‐L‐Leu⁷]Didemnin B Determined by NMR Spectroscopy and Refined by MD Calculation

Several homo‐ and heteronuclear two‐dimensional NMR techniques were used to assign all H‐ and C‐resonances of the two conformers A and B of [7‐(N‐methyl‐L‐leucine)]didemnin B. Didemnine is a biologically highly active cytostaticum and immunosuppressivum. The assignment of the aliphatic C‐atoms were done by the inverse H,C‐COSY with TOCSY transfer which connects complete proton spin systems and represents them on C‐atoms. The structure of both conformers (A and B) in (D₆)DMSO solution was derived from homo‐ and heteronuclear couplings (J), temperature dependencies of NH protons, and NOE effects. Distances determined from the latter were used for refinements by restrained MD calculations using the GROMOS program. The solution structure of [Me‐L‐Leu⁷]didemnin B (A and B) was compared to that of didemnin B. The backbone structure of the macrocyclic ring and of the linear side‐chain moiety are very similar in conformer A and didemnin B, though the Ist¹‐Hip² region of the ring is slightly ex tended in conformer A. This may be caused by the influence of the Me‐L‐Leu⁷ residue in A and may be responsible for its reduced biological activity in comparison to didemnin B. The more weakly populated conformer B exhibits a βVI turn in the linear side‐chain moiety.