Conformational changes in the cyclic undecapeptide cyclosporin induced by interaction with metal ions. An FTIR study

Cyclosporin Structure and Permeability: From A to Z and Beyond

Cyclosporins are natural or synthetic undecapeptides with a wide range of actual and potential pharmaceutical applications. Several members of the cyclosporin compound family have remarkably high passive membrane permeabilities that are not well-described by simple structural metrics. Here we review experimental studies of cyclosporin structure and permeability, including cyclosporin-metal complexes. We also discuss models for the conformation-dependent permeability of cyclosporins and similar compounds. Finally, we identify current knowledge gaps in the literature and provide recommendations regarding future avenues of exploration.

Structural Flexibility of Cyclosporine A Is Mediated by Amide Cis-Trans Isomerization and the Chameleonic Roles of Calcium

Falling outside of Lipinski's rule of five, macrocyclic drugs have accessed unique binding sites of their target receptors unreachable by traditional small molecules. Cyclosporin(e) A (CycA), an extensively studied macrocyclic natural product, is an immunosuppressant with undesirable side effects such as electrolytic imbalances. In this work, a comprehensive view on the conformational landscape of CycA, its interactions with Ca²⁺, and host-guest interactions with cyclophilin A (CypA) is reported through exhaustive analyses that combine ion-mobility spectrometry-mass spectrometry (IMS-MS), nuclear magnetic resonance (NMR) spectroscopy, distance-geometry modeling, and NMR-driven molecular dynamics. Our IMS-MS data show that CycA can adopt extremely compact conformations with significantly smaller collisional cross sections than the closed conformation observed in CDCl₃. To adopt these conformations, the macrocyclic ring has to twist and bend via cis-trans isomerization of backbone amides, and thus, we termed this family of structures the "bent" conformation. Furthermore, NMR measurements indicate that the closed conformation exists at 19% in CD₃OD/H₂O and 55% in CD₃CN. However, upon interacting with Ca²⁺, in addition to the bent and previously reported closed conformations of free CycA, the CycA:Ca²⁺ complex is open and has all-trans peptide bonds. Previous NMR studies using calcium perchlorate reported only the closed conformation of CycA (which contains one cis peptide bond). Here, calcium chloride, a more biologically relevant salt, was used, and interestingly, it helps converting the cis-MeLeu9-MeLeu10 peptide bond into a trans bond. Last, we were able to capture the native binding of CycA and CypA to give forth evidence that IMS-MS is able to probe the solution-phase structures of the complexes and that the Ca²⁺:CycA complex may play an essential role in the binding of CycA to CypA.

1H and 13C-NMR and molecular dynamics studies of cyclosporin a interacting with magnesium(II) or cerium(III) in acetonitrile. Conformational changes and cis-trans conversion of peptide bonds

Cyclosporin A (CsA) is an important drug used to prevent graft rejection in organ transplantations. Its immunosuppressive activity is related to the inhibition of T-cell activation through binding with the proteins Cyclophilin (Cyp) and, subsequently, Calcineurin (CN). In the complex with its target (Cyp), CsA adopts a conformation with all trans peptide bonds and this feature is very important for its pharmacological action. Unfortunately, CsA can cause several side effects, and it can favor the excretion of calcium and magnesium. To evaluate the possible role of conformational effects induced by these two metal ions in the action mechanism of CsA, its complexes with Mg(II) and Ce(III) (the latter as a paramagnetic probe for calcium) have been examined by two-dimensional NMR and relaxation rate analysis. The conformations of the two complexes and of the free form have been determined by restrained molecular dynamics calculations based on the experimentally obtained metal-proton and interproton distances. The findings here ratify the formation of 1:1 complexes of CsA with both Mg(II) and Ce(III), with metal coordination taking place on carbonyl oxygens and substantially altering the peptide structure with respect to the free form, although the residues involved and the resulting conformational changes, including cis-trans conversion of peptide bonds, are different for the two metals.

Solution conformations of cyclosporins and magnesium-cyclosporin complexes determined by vibrational circular dichroism

Vibrational circular dichroism (VCD) spectroscopy was used to investigate the solution conformations of cyclosporins A, C, D, G, and H in CDCl(3), in the amide I and NH/OH-stretching regions, and their corresponding magnesium complexes in CD(3)CN, in the amide I region. VCD spectra are sensitive to the chiral arrangement of Cdbond;O and NH bonds in this cyclic undecapeptide. Calculations of molecular geometries, as well as IR and VCD intensities of model cyclosporin fragments that include the intramolecular hydrogen bonds of the crystal conformations of cyclosporins A and H (CsA and CsH), were carried out at the density functional theory (DFT; BPW91 functional/6-31G* basis set) level. The good agreement between IR and VCD spectra from experiment and DFT calculations provides evidence that the crystal conformation of CsA is dominant in CDCl(3) solution; CsH, however, assumes both an intramolecularly hydrogen-bonded crystal conformation and more open forms in solution. Comparisons of the experimental and calculated VCD spectra in the NH/OH-stretching region of the noncomplexed cyclosporins indicate that conformers with both free and hydrogen-bonded NH and OH groups are present in solution. Differences between the IR and VCD spectra for the metal-free and magnesium-complexed cyclosporins are indicative of strong interactions between cyclosporins and magnesium ions.

Studies of the interaction of potassium(I), calcium(II), magnesium(II), and copper(II) with cyclosporin A

Metal ion binding properties of the immunosuppressant drug cyclosporin A have been investigated. Complexation studies in acetonitrile solution using 1H NMR and CD spectroscopy yielded 1:1 metal-peptide binding constants (log(10)K) for potassium(I), <1, magnesium(II), 4.8+/-0.2, and calcium(II), 5.0+/-1.0. The interaction of copper(II) with cyclosporin A in methanol was investigated with UV/visible and electron paramagnetic resonance (EPR) spectroscopy. No complexation of copper(II) was observed in neutral solution. In the presence of base, monomeric copper(II) complexes were detected. These results support the possibility that cyclosporin A has ionophoric properties for biologically important essential metal ions.