Pseudomerohedrally twinned monoclinic structure of unfolded free nona-ctin: Comparative analysis of its large conformational change upon encapsulation of alkali metal ions

The title compound, C₄₀H₆₄O₁₂, crystallizes in a pseudo-merohedrally twinned primitive monoclinic cell with similar contributions of the two twin components. There are two symmetry-independent half-mol-ecules of nona-ctin in the asymmetric unit. Each mol-ecule has a pseudo-S 4 symmetry and resides on a crystallographic twofold axis; the axes pass through the mol-ecular center of mass and are perpendicular to the plane of the macrocycle. The literature description of the room-temperature structure of nona-ctin as an order-disorder structure in an ortho-rhom-bic unit cell is corrected. We report a low-temperature high-precision ordered structure of free nona-ctin 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.