Australian chemists have kinetically trapped a guest molecule inside a molecular cavity. 

Max Crossley and colleagues at the University of Sydney have designed a rigid porphyrin-based host which encapsulates a dicarboxylic acid in its chiral cavity. The host, a dihydroxotin(IV) porphyrin, has two internal and two external guest interaction sites. Even though the external sites are more accessible, on addition of up to one mole equivalent of dicarboxylic acid, the acid ditopically binds to the sites inside the cavity.

The researchers used ¹H NMR spectroscopy to decipher the mechanism of binding. The dicarboxylic acid binds monotopically within minutes and no free acid is detected. The time taken for the dicarboxylic acid to become ditopically fixed within the cavity increases with increasing chain length. For example, for malonic acid (3 carbon atoms in a chain) this takes less than 2 minutes whereas for adipic acid (6 carbon atoms in a chain) this takes around 30 hours. 

Crossley explains how the mechanism of the internal cavity binding of the carboxylic acid involves the kinetic trapping of a H-bonded guest molecule - a pre-equilibria H-bonded complex converts to an ester-like tin (IV) carboxylate on disassociation of a water molecule.

'This shows how a cavity effect can be enhanced,' explained Crossley, 'this process may be extended to the generation of intricate self-assembled complexes with association constants far in excess of those available through the use of labile interactions alone.' 

'The findings have relevance to enzyme reactions where binding selectively is reliant on synergistic binding interaction,' said Crossley. 

Alison Stoddart