Peptide conformations. Part 31. The conformation of cyclosporin a in the crystal and in solution

Abstract

The all-light-atom X-ray crystal-structure analysis of cyclosporin A (l), a cyclic undecapeptide containing seven N-methylated amino acids, reveals a conformation very similar to that of the previously analysed iodo derivative which is characterised by a twisted P-pleated sheet involving the residues Me-Val-1 1, MeBmt-1, Abu-2, Sar-3, MeLeu-4, Val-5, MeLeu-6, and Ala-7. The@-bend at Sar-MeLeu-4 is of type II', and the loop of the residual amino acids involves a cis-peptide bond between MeLeu-9 and MeLeu-10. The NH proton of D-Ala-8 closes a $-bend with a H-bond to the MeLeu-6 CO group. The crystal was grown from acetone. A closely similar backbone conformation in apolar solvents such as CDCl, or C6D6 has been derived from the interpretation of the NMR spectral parameters (homo-and heteronuclear NOE effects, coupling constants, chemical-shift values of the C-, H-, and N-atoms). A minor variation in the backbone conformation between crystal and solution is observed in the region of o-Ala-8, where in solution a 3-center H-bond is established between the NH of D-Ala-8 und the carbonyl 0-atoms of both MeLeu-6 ($-turn) and o-Ala-8 (C5-bend). A recently proposed technique to identify intramolecular H-bond via heteronuclear NOE from NH proton to carbonyl C-atoms is critically analysed. The main difference between crystal and solution conformations lies in the orientation of the side chains of the unusual amino acid MeBmt kl = f60" in solution, -168" in the crystal) and of MeLeu-10 k1 = -60" in solution, +60' in the crystal). The differences in crystal and solution are caused by the break ofthe intermolecular H-bond of the OH group of MeBmt on dissolution of the crystal. The bifurcated H-bond ofo-Ala-8 twists the backbone in this region. Molecular modeling demonstrates that this is the origin of the change in the side chain conformation of MeLeu-10. The intramolecular flexibility in the crystal indicated by the thermal parameters obtained from the X-ray refinement, and in solution by an analysis of spin-lattice relaxation times in the NMR experiments, indicate a fairly rigid backbone and fixed conformations for all the side chains except for that of Abu-2 and the distal atoms of MeBmt.

Solvent influence on the conformation of cyclosporin. An FT-IR study

“…Against this background, differences between the spectra in CCl, and CDCI, solutions may be ascribed to CDCI, competing with N-H in the C,,=O +-H-N, and (or) C6=0 +-H-N, H-bonds. This effect would account for the minor (6%) conformer inferred from the NMR spectra in CDCI, (2).…”

Peptidylproline cis‐trans‐isomerases: immunophilins

“…Free CsA in the crystal state consists of the cycle which forms a twisted p sheet with a 11'-type turn, and four intramolecular hydrogen bonds between free amides (Loosli et al, 1985). In the bound state, CsA has all the amide bonds in the trans form, the intramolecular hydrogen bonds are broken and the inside of the molecule is flipped over to the outside.…”

Interaction of cyclosporin A with an Fab fragment or cyclophilin Affinity measurements and time‐dependent changes in binding

“…The structure of CS complexed to cyclophilin (CYP), its binding protein in vivo, has also been obtained both by X-ray crystallography [2] and nuclear magnetic resonance [3,4]. The conformation of CS in the two complexes is similar and totally different from its conformation in an isolated form as determined by X-ray and NMR analyses in organic solvents [5,6]. A striking difference between isomers is that the MeLeu9-MeLeu 1° peptide bond is cis in isolated CS and trans in complexed CS.…”