Resumen de Experimental and theoretical studies of a naturally occurring non-oligomeric steroidal supramolecular zipper
D. A. Mulholland, S. E. Drewes, O. Q. Munro, N. R. Crouch, K. du Toit
Methanol extraction of the bulbs of Ornithogalum tenuifolium afforded a novel crystalline steroidal sapogenin (25R)-5β-spirostane-1β,3α-diol (compound 1). The structure and stereochemistry, but not the enantiomer, of this compound was unambiguously assigned using X-ray diffraction and multidimensional 1H and 13C NMR data. The crystal structure of 1 (monoclinic space group P21) is both novel and remarkable in that the extended lattice or “supramolecular” structure comprises an unusual one-dimensional hydrogen-bonded polymer with 21 symmetry (the polymer axis is co-linear with the b-axis of the unit cell). Each ladder-like hydrogen-bonded polymer stacks in an interlocking fashion with a neighbouring polymer along the c-axis of the unit cell. The self-recognition by hydrogen-bonding that leads to the formation of the polymer is characterized by discrete 10-membered supramolecular rings. Moreover, the steroid units of the one-dimensional polymer interlock perfectly to form a supramolecular zipper. The newly-discovered supramolecular synthon of the zipper has been elaborated on due to its possible use in crystal engineering applications. The supramolecular zipper system has also been simulated in vacuo using semi-empirical (AM1) quantum mechanics techniques, which correctly reproduce not only the experimental conformation but also the hydrogen-bonded extended structure of the compound. The simulations showed that formation of the supramolecular zipper structure of 1 most likely reflects charge-complementarity of its hydrogen-bonded hydroxy groups and that the electrostatically driven process is characterized by a favourable and cooperative in vacuo enthalpy of association. A further hallmark of the self-assembly of 1 was the lowering of the mean vibrational frequency of the hydrogen-bonded hydroxy groups upon formation of the zipper.
