Study of the conformation of cyclosporine in aqueous medium by means of monoclonal antibodies

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Abstract
The three‐dimensional structure of the immunosuppressive cyclic peptide cyclosporine (Cs), determined in crystal by X‐ray analysis and in solution in aprotic solvents by n.m.r., differs mainly by the orientation of the 7 carbon side chain of residue 1. Because of its poor solubility in water, the conformation of Cs in aqueous medium cannot be studied by n.m.r. methods, which require concentrations of the substance of the order of milligram/mL but can be analyzed by immunochemical methods in which concentrations in the nanogram/mL range are detected. In the present study, the ability of a series of monoclonal antibodies (McAbs) raised against Cs to recognize different parts of residue 1 of Cs was determined from the cross‐reactivity of different Cs‐analogues modified in residue 1. The results show that when Cs is dissolved in aqueous buffer, the terminal atoms of residue 1 side chain are not available for binding to antibodies recognizing the face of the molecule defined by residues 1, 2, 3, 10, 11, suggesting that the chain is probably folded back under the molecule, as observed in the crystal structure. Binding of McAbs to Cs was also affected by conformational modifications of the peptide ring that occur in some Cs‐analogues. The results illustrate the potential of McAbs for probing the conformation of Cs‐derivatives for which no structural data are available.