Reinvestigation of the Conformation of Cyclosporin A in Chloroform

New investigations of cyclosporin A in CDCl₃ have been performed to obtain additional and more accurate distance restraints than utilized in our previous studies of cyclosporin A. Build-up rates at 600 MHz using 6 different mixing times at low temperatures (252.5 K) were determined and transformed into distances using the two-spin approximation. With the new distance restraints in the MD simulations using the GROMOS package, we can unambiguously conclude the presence of a βII′-turn. The new structure resembles the X-ray structure more than the structure previously determined, especially regarding the orientation of the MeBmt side chain. In the new structure and in the solid state, the side chain is folded over the backbone (although there are substantial differences in the χ₁ torsion), in contrast to the old structure, where the side chain is extended away from the backbone.