Chemistry

Accommodating Hosts

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Science  28 Mar 2014:
Vol. 343, Issue 6178, pp. 1407
DOI: 10.1126/science.343.6178.1407-c
CREDIT: T. SAWADA ET AL., JOURNAL OF THE AMERICAN CHEMICAL SOCIETY (10 MARCH 2014) © 2014 AMERICAN CHEMICAL SOCIETY

The binding of smaller, more rigid guest molecules by larger, more flexible host biomolecules (such as substrates and enzymes) can lead to induced fit: changes in the host structure to accommodate the guest. However, the binding of two large biomolecules can involve structural changes in both partners and is called mutually induced fit. Sawada et al. report a synthetic system in which two small molecules both act as hosts and make structural changes during binding. One host, calix[4]arene, has four phenyl rings that can form a conformer that has a cone-shaped binding surface, but this structure is in rapid equilibrium with other conformers created as the phenyl rings flip down. The other host was formed from a triangular ligand L (formed from three pyridine and pyrimidine rings bound to a central pyrimidine ring) reacting with a Pd complex in aqueous solution. Two structures formed, mainly Pd6L3 triagonal pyramids (94%) but also Pd8L4 tetragonal pyramids (6%). When these two hosts were mixed in solution, the calix[4]arene shifted entirely to the cone structure, and the second host shifted entirely to the minority tetragonal pyramid structure to create an extensive binding surface. Furthermore, this host trapped a nitrate ion, which neither host alone could do.

J. Am. Chem. Soc. 10.1021/ja500376x (2014).

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