text: Supramolecular catalysts can accelerate reactions not only by placing the two reactants in close proximity but also by stabilizing the transition state of the reaction and reducing activation energy. While this fundamental principle of catalysis is common in small molecule or heterogeneous catalysts, supramolecular catalysts however has a difficult time utilizing the concept due to their often rigid structures. Unlike enzymes that can change shape to accommodate the substrates, supramolecules do not have that kind of flexibility and so rarely achieve sub-angstrom adjustment required for perfect transition state stabilization.
  An example of catalysts of this type is Sander's porphyrin trimer. A Diels Alder reaction between two pyridine functionalized substrates normally yield a mixture of endo and exo products. In the presence of the two catalysts, however, complete endo selectivity or exo selectivity could be obtained. The underlying cause of the selectivity is the coordination interaction between pyridine and the zinc ion on porphyrin. Depending on the shape of the catalysts, one product is preferred over the other.