Peptide conformations. Part 30. Assignment of the ¹H‐, ¹³C‐, and ¹⁵N‐NMR spectra of cyclosporin A in CDCl₃ and C₆D₆ by a combination of homo‐ and heteronuclear two‐dimensional techniques

The ¹H‐, ¹³C‐, and ¹⁵N‐NMR spectra of the immunosuppressive cyclic undecapeptide cyclosporin A (1) have been analyzed at 300 MHz in CDCl₃, C₆D₆, and mixtures of these solvents. A combination of different homo‐ and heteronuclear two‐dimensional NMR techniques enable complete assignment of all H‐, C‐ and 4 N‐signals. Recognition of the proton spin systems has been achieved via ¹H,¹H–COSY and double‐quantum‐¹H‐NMR spectroscopy. NOESY spectra yield some sequence assignments, but two techniques using coupling across amide bonds have been applied to get independent assignments of all amino acids in the sequence: (i) An ¹H,¹H‐COSY spectrum optimized for small coupling constants enables the detection of long‐range couplings from N‐methyl groups to both α‐protons attached to that amide bond. (ii) An ¹H, ¹³C‐COSY spectrum optimized for C,H‐long‐range couplings (J = 5 to 10 Hz) to the eleven CO groups again yields coupling to both α‐protons attached to that amide bond. Additionally these two experiments yield the assignment of N‐methyl protons and carbonyl C‐atoms. Normal and relayed ¹H,¹³C‐COSY in both solvents have been applied to assign all C‐atoms via their directly attached and remote protons. An ¹H,¹³C‐COLOC spectrum at 500 MHz in CDCl₃, which uses H,C‐long‐range couplings confirms the assignment of all proton spin systems as well as the C‐signals of each individual amino acid. Ambiguities in the assignment of the C(δ)'s of MeLeu have thus been removed. An ¹H,¹⁵N‐COSY spectrum enables the assignment of the 4 NH N‐atoms.