Host & Guest Chemistry

by mari

Vaguely inspired by this prompt.

As a molecular host, finding the right guest can be difficult. It’s all a high-stakes game of chance and proximity.

The fabled selectivity of the molecule cyclodextrin for the benzene ring reminded one  epigrammatic duo of scientists of the lock-and-key model for enzymatic selectivity. So they named the field host-guest chemistry.

The thing about chemical houseguests? Perfect complementarity.
Cyclodextrin is the paladin of all molecular hosts, and benzene, the the ideal guest.

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Image: Benzene makes for a better houseguest than Goldilocks.

If, like cyclodextrin and benzene, two molecules are destined for the sacred union of host and guest, not even the mirror image of the guest can pair with the host.

Imagine that you had an identical twin who, of the two of you, was the only one welcome in your parents’ house. It’s the beautiful logic of enantiomers, molecules that are non-superimposable mirror images of each other. This logic can be summed up thusly: A left-hand foot cannot be placed in a right-hand shoe.

Even if the shoe fits–even if the guest conforms to its molecular host–electrostatic binding sites of host and guest must also align. And if charges are not complementary, the guest will be summarily repulsed by the host.

But if noncovalent interactions are successful, our guest is out of solution for the night!

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*The epigrammatic duo, Drs. Jane and Donald Cram, were actually married, in another instance of perfect complementarity. While only one grainy page of the PDF is available to the unsubscribed, their foundational 1974 paper on host-guest chemistry is archived here on Science

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