Isolation and Stabilization of a Pheromone in Crystalline Molecular Capsules

The active monomer form of the male-produced
pheromone of the Mediterranean
fruit fly can be isolated selectively from its equilibrating trimer
species by encapsulation within a calixarene pocket built into a hydrogen-bonded
framework from guanidinium 4-sulfocalix[4]­arene. Encapsulation of
the Δ1-pyrroline guest significantly perturbs the
assembly of the quasihexagonal two-dimensional guanidinium-sulfonate
network of the guest-free framework, to the extent that guanidinium
ions are excluded from some sites to accommodate the steric requirements
of the guest. Nonetheless, single crystal X-ray diffraction reveals
the preservation of a layered structrure in which the calixarene capsules
stack in an antiparallel configuration. These observations illustrate
that the binding of the pheromone monomer by the calixarene is sufficiently
strong to overcome the loss of guanidinium-sulfonate hydrogen bonds,
which is corroborated by the strong binding constants measured in
solution. The solid-state encapsulation stabilizes the otherwise volatile
unstable monomer form, suggesting an effective strategy for the storage,
application, and controlled release of an important agricultural adjuvant.