Release of the Prays oleae pheromone as a consequence of supramolecular structure:: study of the β-cyclodextrin-(Z)-tetradec-7-en-1-al complex by X-ray crystallography and NMR spectroscopy in the solid state and in solution

The structure of the beta-cyclodextrin-(Z)-tetradec-7-en-1-al complex in aqueous solution and in the solid state, as well as the release profile of the (Z)-tetradec-7-en-1-al (sex pheromone of the olive pest Prays oleae) from the solid complex was investigated, in an effort to correlate the supramolecular structure with the macroscopic property of spontaneous liberation of the pheromone. It was observed that in solution a 2 : 1 host : guest complex prevails, having the guest in a curled configuration. In the crystal structure of the complex, two beta-CD molecules forming head-to-head dimers and packed in channels enclose one guest molecule whose methyl terminal aliphatic chain curls at the cis-double bond and runs along the intradimer interface. In the space between host molecules there is also entrapped an additional pheromone molecule, also visible in the IR spectra, which is heavily disordered. The guest inside the cavity is disordered over two sites and exhibits mobility, especially at the methyl and carbonyl end-groups, which is also confirmed by solid state NMR experiments. Thus, there are two types of guest molecules in the crystalline complex, one inside the beta-CD cavity and another trapped and held loosely outside the cavity. The release behavior, studied by NMR, shows that the "outside" pheromone is liberated from the solid initially at a fast rate, which reaches very low levels when almost half of the guest molecules have been released. The other half, molecularly encapsulated in the beta-CD cavity, is well stabilized.