Observation of Conformational Exchange in Cyclosporin in Media of Varying Polarity by NMR Spectroscopy

The molecule of peptide cyclosporin A experiences chemical exchange in polar solvents. In apolar media, such as chloroform or benzene, this transformation is suppressed, but still leads to formation of a small fraction of a minor conformer. To elucidate the nature of this phenomenon, the peptide was dissolved in mixed solvents chloroform-DMSO and water-DMSO. Analysis of H-1 nuclear magnetic resonance and two-dimensional exchange spectroscopy spectra showed that the conformational exchange proceeds at a low rate of 10(-1) s(-1) at the room temperature and involves passing over a high free energy barrier. Thus the situation resembles the exchange process in chloroform, associated with cis-trans isomerization of peptide bonds, but in the presence of DMSO transformation occurs at several sites independently, and the energy difference between arising conformers is small, 10(2)-10(3) kJ/mol.