Conformation of MeAla6‐cyclosporin A by NMR: Relationship of sidechain orientation of the MeBmt‐l, MeLeu‐9, and MeLeu‐10 residues to immunosuppressive activity

MeAla6‐cyclosporin A (MeAla6‐CsA) is a unique CsA analog that shows weak immunosuppressive activity and yet binds strongly to the proposed cytosolic protein receptor, cyclophilin (CyP). Preliminary ¹H NMR data showed significant chemical shift differences between spectra of MeAla6‐CsA and CsA, suggesting different preferred conformations. A more detailed study, however, revealed that the backbone conformations of the two molecules are essentially identical, and that the differences can be accounted for, principally, by the sidechain motions of the MeBmt‐1, MeLeu‐9, and ‐10 residues. ROE and coupling constant data show that in MeAla6‐CsA, the preferred χ₁ rotamers for MeLeu‐9 and ‐10 are + 180° (T), whereas in CsA there is a more even distribution of rotamer populations for MeLeu‐10, and a preferred – 60° (G⁻) χ₁ rotamer for MeLeu‐9. Similar data argue that the sidechain of MeBmt‐1 is more restricted in its motion in MeAla‐CsA than in CsA. Temperature studies suggest that these preferred rotamers for MeAla6‐CsA may increase the stability of the hydrogen bond between NH(7) and CO(II), but prevent particular residues, especially the essential MeBmt‐I sidechain, from adopting orientations required to elicit immunosuppressive activity. The significant changes observed in the preferred orientations for the side‐chains of the MeBmt‐1, MeLeu‐9, and MeLeu‐10 residues in MeAla6‐CsA argue that the particular orientations which they assume in CsA are not essential for cyclophilin binding.