Pseudomerohedrally twinned monoclinic structure of unfolded 'free' nonactin: comparative analysis of its large conformational change upon encapsulation of alkali metal ions.

The title compound, C40H64O12, crystallizes in a pseudomerohedrally twinned primitive monoclinic cell with similar contributions of the two twin components. There are two symmetry-independent half-molecules of nonactin in the asymmetric unit. Each molecule has a pseudo-S4 symmetry and resides on a crystallographic twofold axis; the axes pass through the molecular center of mass and are perpendicular to the plane of the macrocycle. The literature description of the room-temperature structure of nonactin as an order-disorder structure in an orthorhombic unit cell is corrected. We report a low-temperature high-precision ordered structure of ;free' nonactin that allowed for the first time precise determination of its bond distances and angles. It possesses an unfolded and more planar geometry than its complexes with encapsulated Na+, K+, Cs+, Ca2+ or NH4+ cations that exhibit more isometric overall conformations.