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

¹ Department of Chemistry and the NMR Centre, University of Siena

^* To whom correspondence should be addressed. E-mail: valensin{at}unisi.it.

Submitted on September 9, 2005
Revised on October 3, 2005
Accepted on 21 October 2005

	Abstract

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 favour the excretion of calcium and magnesium. In order 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 2D 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 inter-proton distances. The present findings 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.

Key Words: Cyclosporin, NMR, metal-complexes, molecular dynamics